Restore $SAFE after running a safe mode test (if possible).

Ruby 2.6 will make $SAFE a global option (instead of per-thread) and
will allow the $SAFE level to be downgraded.

https://bugs.ruby-lang.org/issues/14250

(cherry picked from commit d84b8ae)

test/test_utils.rb

@@ -62,18 +62,34 @@ def safe_test(options = {})
       thread = Thread.new do
         orig_diff = Minitest::Assertions.diff
 
-        $SAFE = options[:level] || 1 if available
-
-        # Disable the use of external diff tools during safe mode tests (since
-        # safe mode will prevent their use). The initial value is retrieved
-        # before activating safe mode because the first time
-        # Minitest::Assertions.diff is called, it will attempt to find a diff
-        # tool. Finding the diff tool will also fail in safe mode.
-        Minitest::Assertions.diff = nil
+        if available
+          orig_safe = $SAFE
+          $SAFE = options[:level] || 1
+        end
         begin
-          yield
+          # Disable the use of external diff tools during safe mode tests (since
+          # safe mode will prevent their use). The initial value is retrieved
+          # before activating safe mode because the first time
+          # Minitest::Assertions.diff is called, it will attempt to find a diff
+          # tool. Finding the diff tool will also fail in safe mode.
+          Minitest::Assertions.diff = nil
+          begin
+            yield
+          ensure
+            Minitest::Assertions.diff = orig_diff
+          end
         ensure
-          Minitest::Assertions.diff = orig_diff
+          if available
+            # On Ruby < 2.6, setting $SAFE affects only the current thread
+            # and the $SAFE level cannot be downgraded. Catch and ignore the
+            # SecurityError.
+            # On Ruby >= 2.6, setting $SAFE is global, and the $SAFE level
+            # can be downgraded. Restore $SAFE back to the original level.
+            begin
+              $SAFE = orig_safe
+            rescue SecurityError
+            end
+          end
         end
       end