log test: don't assume a fixed set of letters for $GOROOT

Fixes #27.

R=r
https://golang.org/cl/152075

Author: rsc

src/pkg/log/log_test.go

@@ -18,7 +18,7 @@ const (
 	Rtime		= `[0-9][0-9]:[0-9][0-9]:[0-9][0-9]`;
 	Rmicroseconds	= `\.[0-9][0-9][0-9][0-9][0-9][0-9]`;
 	Rline		= `[0-9]+:`;
-	Rlongfile	= `/[A-Za-z0-9_/\-]+\.go:` + Rline;
+	Rlongfile	= `.*/[A-Za-z0-9_\-]+\.go:` + Rline;
 	Rshortfile	= `[A-Za-z0-9_\-]+\.go:` + Rline;
 )