cmd/asm/internal/asm: abort end to end test if assembly failed

If errors are encountered during assembly, do not attempt to perform verification.
In this case the output is unlikely to be correct and all verification fails, which
means the real issue gets lost in the noise.

Change-Id: I62c1bf09fa025b0df4c06f0bfa424fb5d328184b
Reviewed-on: https://go-review.googlesource.com/c/go/+/731920
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Joel Sing <joel@sing.id.au>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/cmd/asm/internal/asm/endtoend_test.go

@@ -199,6 +199,11 @@ Diff:
 	}
 	obj.Flushplist(ctxt, pList, nil)
 
+	if !ok {
+		// If we've encountered errors, the output is unlikely to be sane.
+		t.FailNow()
+	}
+
 	for p := top; p != nil; p = p.Link {
 		if p.As == obj.ATEXT {
 			text = p.From.Sym