[gopls-release-branch.0.17] gopls/internal/cache/testfuncs: fix crash with *error argument

The test detection logic incorrectly assumes that named types have a
non-nil package, which is not true for Error (as we've encountered
numerous times). Add the missing nil check.

Fixes golang/go#70927

Change-Id: Ibdf483304b53ce219123e820d74acd4f9d12d710
Reviewed-on: https://go-review.googlesource.com/c/tools/+/637815
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
(cherry picked from commit dcc725c)
Reviewed-on: https://go-review.googlesource.com/c/tools/+/637736

Author: findleyr

gopls/internal/cache/testfuncs/tests.go

@@ -281,7 +281,7 @@ func testKind(sig *types.Signature) (*types.TypeName, bool) {
 	}
 
 	named, ok := ptr.Elem().(*types.Named)
-	if !ok || named.Obj().Pkg().Path() != "testing" {
+	if !ok || named.Obj().Pkg() == nil || named.Obj().Pkg().Path() != "testing" {
 		return nil, false
 	}
 

gopls/internal/test/integration/workspace/packages_test.go

@@ -124,6 +124,7 @@ func TestFoo2(t *testing.T)
 package foo
 import "testing"
 func TestFoo(t *testing.T)
+func Issue70927(*error)
 
 -- foo2_test.go --
 package foo_test