cmd/compile/internal/gc: make tests run faster

TestAssembly takes 20s on my machine,
which is too slow for normal operation.
Marking as -short has its dangers (#17472),
but hopefully we'll soon have a builder for that.

All the SSA tests are hermetic and not time sensitive
and can thus be run in parallel.
Reduces the cmd/compile/internal/gc test time during
all.bash on my laptop from 42s to 7s.

Updates #17751

Change-Id: Idd876421db23b9fa3475e8a9b3355a5dc92a5a29
Reviewed-on: https://go-review.googlesource.com/32585
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/asm_test.go

@@ -21,6 +21,9 @@ import (
 // TestAssembly checks to make sure the assembly generated for
 // functions contains certain expected instructions.
 func TestAssembly(t *testing.T) {
+	if testing.Short() {
+		t.Skip("slow test; skipping")
+	}
 	testenv.MustHaveGoBuild(t)
 	if runtime.GOOS == "windows" {
 		// TODO: remove if we can get "go tool compile -S" to work on windows.

src/cmd/compile/internal/gc/ssa_test.go

@@ -16,9 +16,11 @@ import (
 // TODO: move all these tests elsewhere?
 // Perhaps teach test/run.go how to run them with a new action verb.
 func runTest(t *testing.T, filename string) {
+	t.Parallel()
 	doTest(t, filename, "run")
 }
 func buildTest(t *testing.T, filename string) {
+	t.Parallel()
 	doTest(t, filename, "build")
 }
 func doTest(t *testing.T, filename string, kind string) {