Missing error checks for os.Setenv in parse_test.go

[antcs]

Actual behavior A clear and concise description of what the bug is.

Currently errors are ignored for calls to os.Setenv() in parse_test.go. Example:

mock/mockgen/parse_test.go

Line 198 in b76a85f

os.Setenv("GOPATH", goPath)

Expected behavior A clear and concise description of what you expected to
happen.

The errors should be checked and reported properly. If the error is unequal nil the error should be logged via t.Errorf(). I opened a pull request to check the errors: #472