runtime: remove TestCollisions from -short

Takes a bit too long to run it all the time.

Fixes #17217
Update #17104

Change-Id: I4802190ea16ee0f436a7f95b093ea0f995f5b11d
Reviewed-on: https://go-review.googlesource.com/29751
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/hash_test.go

@@ -683,6 +683,9 @@ func BenchmarkUnalignedLoad(b *testing.B) {
 }
 
 func TestCollisions(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Skipping in short mode")
+	}
 	for i := 0; i < 16; i++ {
 		for j := 0; j < 16; j++ {
 			if j == i {