cmd/compile: delete temporary files when test finishes

cmd/compile: delete temporary files when test finishes

I noticed some files prefixed with ssa_fg_tmp in the /tmp folder of
the s390x builder. runGenTest (a helper for TestGenFlowGraph) wasn't
deleting its temporary files. The distinct prefix made this easy to
figure out.

Change-Id: If0d608aaad04a414e74e29f027ec9443c626e4eb
Reviewed-on: https://go-review.googlesource.com/78475
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>