[dev.typeparams] cmd/compile: do not skip TestUnifiedCompare in short mode

Instead, just testing the runtime package in short mode instead of std.
So trybot can help catching any mistake earlier.

Change-Id: I5bc859a59afa13a10d436a798258ac154c7d27b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/330836


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>