errors/fmt: fix tests that assume GOPATH layout

LUCI doesn't check the source out into a GOPATH-style layout like
trybots do. Fix the tests that relied on that.

Change-Id: I6ebaf1dce5cf831c63743f64acee332dacb7f102
Reviewed-on: https://go-review.googlesource.com/c/exp/+/506297
Run-TryBot: Heschi Kreinick <heschi@google.com>
Auto-Submit: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

errors/fmt/errors_test.go

@@ -141,7 +141,7 @@ func TestErrorFormatter(t *testing.T) {
 		fmt: "%+v",
 		want: "something:" +
 			"\n    golang.org/x/exp/errors/fmt_test.TestErrorFormatter" +
-			"\n        .+/golang.org/x/exp/errors/fmt/errors_test.go:98" +
+			"\n        .+/fmt/errors_test.go:98" +
 			"\n    something more",
 		regexp: true,
 	}, {

errors/fmt/format_example_test.go

@@ -40,8 +40,8 @@ func Example_formatting() {
 }
 
 func stripPath(s string) string {
-	rePath := regexp.MustCompile(`( [^ ]*)golang.org`)
-	s = rePath.ReplaceAllString(s, " golang.org")
+	rePath := regexp.MustCompile(`( [^ ]*)/fmt`)
+	s = rePath.ReplaceAllString(s, " golang.org/x/exp/errors/fmt")
 	s = filepath.ToSlash(s)
 	return s
 }