Evaluating the symlinks was slowing down test cache checks.

Fixes #26562
Fixes #26726

	ijt:~/gopath/src/issue26562$ cat foo_test.go
	package foo_test

	import (
		"fmt"
		"os"
		"path/filepath"
		"testing"
	)

	// package and imports snipped
	func TestCache(t *testing.T) {
		tmp := os.TempDir()
		for i := 0; i < 1000000; i++ {
			os.Stat(filepath.Join(tmp, fmt.Sprintf("%d", i)))
		}
	}
	ijt:~/gopath/src/issue26562$ time ~/github/go/bin/go test -count=1
	PASS
	ok      issue26562      9.444s

	real    0m10.021s
	user    0m2.344s
	sys     0m7.835s
	ijt:~/gopath/src/issue26562$ time ~/github/go/bin/go test .
	ok      issue26562      (cached)

	real    0m0.802s
	user    0m0.551s
	sys     0m0.306s

Change-Id: I3ce7f7b68bb5b9e802069f277e79e1ed3c162622
Reviewed-on: https://go-review.googlesource.com/127635
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>