pw_assert_basic: Flush and abort rather than exit

Debuggers break on abort(), but not exit(1). Abort doesn't allow stdout
and stderr to flush, so manually flush them first.

Change-Id: I8510478fe5ebdb27c09519e9e345ec9b3702203d
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/49840
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>