regexp: exclude exhaustive RE2 test from running under race detector.

It is an expensive test to run, and even more so with -race,
and causes timeouts on builders. It is doubtful that it would
find a race that other tests in this package wouldn't, so there
is little loss in excluding it.

Update #5837.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/11568043

src/pkg/regexp/exec_test.go

@@ -67,13 +67,6 @@ func TestRE2Search(t *testing.T) {
 	testRE2(t, "testdata/re2-search.txt")
 }
 
-func TestRE2Exhaustive(t *testing.T) {
-	if testing.Short() {
-		t.Skip("skipping TestRE2Exhaustive during short test")
-	}
-	testRE2(t, "testdata/re2-exhaustive.txt.bz2")
-}
-
 func testRE2(t *testing.T, file string) {
 	f, err := os.Open(file)
 	if err != nil {