cmd/internal/testdir: update errors when filepaths include 'C:\'

Currently on Windows, commands like:

 go test cmd/internal/testdir -run=foo -update_errors

will fail to update the errors because the parsing is
currently confused by the ':' in filepaths that
start with 'C:\', and wrongly thinks that ':' marks
the end of the Go filename.

Instead of finding the first ':', use a regexp
to find what looks to be the end of the Go filename.

Change-Id: I091106da55b8e9e9cf421814abf26a6f8b821af9
Reviewed-on: https://go-review.googlesource.com/c/go/+/524942
Reviewed-by: Russ Cox <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: thepudds

src/cmd/internal/testdir/testdir_test.go

@@ -1276,9 +1276,16 @@ func (test) updateErrors(out, file string) {
 	// Parse new errors.
 	errors := make(map[int]map[string]bool)
 	tmpRe := regexp.MustCompile(`autotmp_\d+`)
+	fileRe := regexp.MustCompile(`(\.go):\d+:`)
 	for _, errStr := range splitOutput(out, false) {
-		errFile, rest, ok := strings.Cut(errStr, ":")
-		if !ok || errFile != file {
+		m := fileRe.FindStringSubmatchIndex(errStr)
+		if len(m) != 4 {
+			continue
+		}
+		// The end of the file is the end of the first and only submatch.
+		errFile := errStr[:m[3]]
+		rest := errStr[m[3]+1:]
+		if errFile != file {
 			continue
 		}
 		lineStr, msg, ok := strings.Cut(rest, ":")