xerrors: fix TestErrorf false positive when top-level dir is renamed

The rePath pattern will typically replace two matches with "/path.":
one with the test's import path, like " /golang.org/x/xerrors_test.",
and a file path like " /Users/gopher/golang.org/x/xerrors/fmt_test.".

The former will always end with "/xerrors_test." as long as the module
path is still "golang.org/x/xerrors" and the TestErrorf test is inside
an xerrors_test package. The latter may not match xerrors.*test if the
top-level directory isn't named "xerrors". But it will always end with
"/fmt_test." as long as the TestErrorf test is still inside the file
named "fmt_test.go".

So look for those two patterns to fix false positive failures when the
git repository is cloned to a custom path without any "xerrors" in it.

Change-Id: I4d95121734337050309c2d989afd8f82430a2f31
Reviewed-on: https://go-review.googlesource.com/c/xerrors/+/609380
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

fmt_test.go

diff --git a/fmt_test.go b/fmt_test.go
index 99d945f..1ee7e6f 100644
--- a/fmt_test.go
+++ b/fmt_test.go
@@ -555,14 +555,14 @@
 
 func (panicValue) String() string { panic("panic") }
 
-var rePath = regexp.MustCompile(`( [^ ]*)xerrors.*test\.`)
-var reLine = regexp.MustCompile(":[0-9]*\n?$")
+var rePath = regexp.MustCompile(`( [^ ]+)\/(xerrors_test|fmt_test)\.`)
+var reLine = regexp.MustCompile(":[0-9]+\n?$")
 
 func cleanPath(s string) string {
 	s = rePath.ReplaceAllString(s, "/path.")
 	s = reLine.ReplaceAllString(s, ":xxx")
-	s = strings.Replace(s, "\n   ", "", -1)
-	s = strings.Replace(s, " /", "/", -1)
+	s = strings.ReplaceAll(s, "\n   ", "")
+	s = strings.ReplaceAll(s, " /", "/")
 	return s
 }