Fix error-prone shell command status check

In the case of the tests, `$?` was not mocked, so it looks like it was
indeed just a coincidence that it worked. Meaning, some other shell
command that somehow was actually executed (so it wasn't stubbed)
succeeded, which caused the `$?.success?` to succeed too.

fastlane_core/lib/fastlane_core/cert_checker.rb

@@ -160,13 +160,13 @@ def self.install_wwdr_certificate(cert_alias)
       import_command = "curl -f -o #{filename} #{url} && security import #{filename} #{keychain}"
       UI.verbose("Installing WWDR Cert: #{import_command}")
 
-      stdout, stderr, _status = Open3.capture3(import_command)
+      stdout, stderr, status = Open3.capture3(import_command)
       if FastlaneCore::Globals.verbose?
         UI.command_output(stdout)
         UI.command_output(stderr)
       end
 
-      unless $?.success?
+      unless status.success?
         UI.verbose("Failed to install WWDR Certificate, checking output to see why")
         # Check the command output, WWDR might already exist
         unless /The specified item already exists in the keychain./ =~ stderr

fastlane_core/spec/cert_checker_spec.rb

@@ -1,5 +1,11 @@
 describe FastlaneCore do
   describe FastlaneCore::CertChecker do
+    let(:success_status) {
+      allow_any_instance_of(String).to receive(:success?).and_return(true)
+
+      ""
+    }
+
     describe '#installed_identies' do
       it 'should print an error when no local code signing identities are found' do
         allow(FastlaneCore::CertChecker).to receive(:installed_wwdr_certificates).and_return(['G1', 'G2', 'G3', 'G4', 'G5', 'G6'])
@@ -65,22 +71,22 @@
       it 'should download the WWDR certificate from correct URL' do
         allow(FastlaneCore::CertChecker).to receive(:wwdr_keychain).and_return('login.keychain')
 
-        expect(Open3).to receive(:capture3).with(include('https://developer.apple.com/certificationauthority/AppleWWDRCA.cer')).and_return("")
+        expect(Open3).to receive(:capture3).with(include('https://developer.apple.com/certificationauthority/AppleWWDRCA.cer')).and_return(["", "", success_status])
         FastlaneCore::CertChecker.install_wwdr_certificate('G1')
 
-        expect(Open3).to receive(:capture3).with(include('https://www.apple.com/certificateauthority/AppleWWDRCAG2.cer')).and_return("")
+        expect(Open3).to receive(:capture3).with(include('https://www.apple.com/certificateauthority/AppleWWDRCAG2.cer')).and_return(["", "", success_status])
         FastlaneCore::CertChecker.install_wwdr_certificate('G2')
 
-        expect(Open3).to receive(:capture3).with(include('https://www.apple.com/certificateauthority/AppleWWDRCAG3.cer')).and_return("")
+        expect(Open3).to receive(:capture3).with(include('https://www.apple.com/certificateauthority/AppleWWDRCAG3.cer')).and_return(["", "", success_status])
         FastlaneCore::CertChecker.install_wwdr_certificate('G3')
 
-        expect(Open3).to receive(:capture3).with(include('https://www.apple.com/certificateauthority/AppleWWDRCAG4.cer')).and_return("")
+        expect(Open3).to receive(:capture3).with(include('https://www.apple.com/certificateauthority/AppleWWDRCAG4.cer')).and_return(["", "", success_status])
         FastlaneCore::CertChecker.install_wwdr_certificate('G4')
 
-        expect(Open3).to receive(:capture3).with(include('https://www.apple.com/certificateauthority/AppleWWDRCAG5.cer')).and_return("")
+        expect(Open3).to receive(:capture3).with(include('https://www.apple.com/certificateauthority/AppleWWDRCAG5.cer')).and_return(["", "", success_status])
         FastlaneCore::CertChecker.install_wwdr_certificate('G5')
 
-        expect(Open3).to receive(:capture3).with(include('https://www.apple.com/certificateauthority/AppleWWDRCAG6.cer')).and_return("")
+        expect(Open3).to receive(:capture3).with(include('https://www.apple.com/certificateauthority/AppleWWDRCAG6.cer')).and_return(["", "", success_status])
         FastlaneCore::CertChecker.install_wwdr_certificate('G6')
       end
     end
@@ -105,7 +111,7 @@
         cmd = %r{curl -f -o (([A-Z]\:)?\/.+) https://www\.apple\.com/certificateauthority/AppleWWDRCAG6\.cer && security import \1 -k #{Regexp.escape(keychain.shellescape)}}
         require "open3"
 
-        expect(Open3).to receive(:capture3).with(cmd).and_return("")
+        expect(Open3).to receive(:capture3).with(cmd).and_return(["", "", success_status])
         expect(FastlaneCore::CertChecker).to receive(:wwdr_keychain).and_return(keychain_name)
 
         allow(FastlaneCore::CertChecker).to receive(:installed_wwdr_certificates).and_return(['G1', 'G2', 'G3', 'G4', 'G5'])