go/types: remove need to enumerate fixedbugs test files

go/types: remove need to enumerate fixedbugs test files

This is a port of CL 244628, updated to move some existing regression
tests into the fixedbugs directory, and to use subtests. Also,
'TestFixed' is renamed to 'TestFixedBugs'.

Change-Id: I43aac3f75f2bd850567d08e8b008d91aeb717064
Reviewed-on: https://go-review.googlesource.com/c/go/+/247904
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>