This makes the tests slightly faster, though the bulk of the time is
still spent building the test programs.

Also run some tests in parallel.

Updates #26470

Change-Id: Ia5ec2b99831d69c426b43dbab80613aa03e705f5
Reviewed-on: https://go-review.googlesource.com/c/153258Reviewed-by: Austin Clements <austin@google.com>