test/fixedbugs: fix bug248 and bug345

test/fixedbugs: fix bug248 and bug345

When test/run script was removed, these two tests
were changed to be executed by test/run.go.
Because errchk does not exit with non-zero status on
errors, they were silently failing for a while.

This change makes 2 things:

1. Compile tested packages in GOROOT/test to match older runner script
   behavior (strictly required only in bug345, optional in bug248)

2. Check command output with "(?m)^BUG" regexp.
   It approximates older `grep -q '^BUG' that was used before.

See referenced issue for detailed explanation.

Fixes #24629

Change-Id: Ie888dcdb4e25cdbb19d434bbc5cb03eb633e9ee8
Reviewed-on: https://go-review.googlesource.com/104095
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>