TestTrimGOPATH fails when vendored

[exp]

Due to using a simple string inequality test this function will fail if the errors package is located in a vendor subdirectory:

--- FAIL: TestTrimGOPATH (0.00s)
        stack_test.go:165: stack_test.go:9: want "github.com/pkg/errors/stack_test.go", got "example.org/test/vendor/github.com/pkg/errors/stack_test.go"

A simple fix is to replace lines 163-166 with the same testFormatRegex function used elsewhere:

want := tt.want
if want != got {
    t.Errorf("%v: want %q, got %q", tt.Frame, want, got)
}

to

testFormatRegexp(t, got, "%s", tt.want)

This also requires changing the line numbers on the rest of the file, so a pull request is incoming shortly. This will also slightly change the error message returned when this test fails but I am not sure if that of any great consequence.