runtime: test change to adapt to new inliner

The new inliner tries to de-prioritize inlining of call sites on panic
paths, e.g. for a call such as the one to "foo" below, the inliner
will use a much lower size threshold when deciding whether to inline,
since the path is very likely to be "cold".

   if mumble() {
      foo()           <<-- here
      panic("bad")
   }

This patch reworks one of the traceback tests is relying on the old
inliner's "inline F everywhere if F inlinable" strategy by tweaking
the code slightly (no change in test functionality).

Change-Id: I83a686b0cc4d94a6cfc63d1e84e45455c1afd5b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/519196
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/runtime/traceback_test.go

@@ -108,7 +108,13 @@ func ttiSigpanic1() (res *ttiResult) {
 		recover()
 	}()
 	ttiSigpanic2()
-	panic("did not panic")
+	// without condition below the inliner might decide to de-prioritize
+	// the callsite above (since it would be on an "always leads to panic"
+	// path).
+	if alwaysTrue {
+		panic("did not panic")
+	}
+	return nil
 }
 func ttiSigpanic2() {
 	ttiSigpanic3()
@@ -118,6 +124,8 @@ func ttiSigpanic3() {
 	*p = 3
 }
 
+var alwaysTrue = true
+
 //go:noinline
 func ttiWrapper1() *ttiResult {
 	var w ttiWrapper