Fix test compat with RubyGems in Ruby 2.5+

Fixes #1161

spec/cli/server_spec.rb

@@ -85,6 +85,10 @@ def mock_file(filename, content = nil)
     mock_file(filename_e) unless filename_e == filename
   end
 
+  before :each do
+    allow(File).to receive(:exist?).and_call_original
+  end
+
   context 'when .yardopts file exists' do
     before :each do
       Registry.yardoc_file = Registry::DEFAULT_YARDOC_FILE

spec/cli/yri_spec.rb

@@ -46,9 +46,10 @@ def print_object(*args) test_stub; super end
   describe "#initialize" do
     it "loads search paths" do
       path = %r{/\.yard/yri_search_paths$}
-      expect(File).to receive(:file?).with(%r{/\.yard/yri_cache$}).and_return(false)
-      expect(File).to receive(:file?).with(path).and_return(true)
-      expect(File).to receive(:readlines).with(path).and_return(%w(line1 line2))
+      allow(File).to receive(:file?).and_call_original
+      allow(File).to receive(:file?).with(%r{/\.yard/yri_cache$}).and_return(false)
+      allow(File).to receive(:file?).with(path).and_return(true)
+      allow(File).to receive(:readlines).with(path).and_return(%w(line1 line2))
       @yri = YARD::CLI::YRI.new
       spaths = @yri.instance_variable_get("@search_paths")
       expect(spaths).to include('line1')
@@ -58,9 +59,10 @@ def print_object(*args) test_stub; super end
     it "uses DEFAULT_SEARCH_PATHS prior to other paths" do
       YARD::CLI::YRI::DEFAULT_SEARCH_PATHS.push('foo', 'bar')
       path = %r{/\.yard/yri_search_paths$}
-      expect(File).to receive(:file?).with(%r{/\.yard/yri_cache$}).and_return(false)
-      expect(File).to receive(:file?).with(path).and_return(true)
-      expect(File).to receive(:readlines).with(path).and_return(%w(line1 line2))
+      allow(File).to receive(:file?).and_call_original
+      allow(File).to receive(:file?).with(%r{/\.yard/yri_cache$}).and_return(false)
+      allow(File).to receive(:file?).with(path).and_return(true)
+      allow(File).to receive(:readlines).with(path).and_return(%w(line1 line2))
       @yri = YARD::CLI::YRI.new
       spaths = @yri.instance_variable_get("@search_paths")
       expect(spaths[0, 4]).to eq %w(foo bar line1 line2)