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Now there are just three programs to compile instead of many,
and repeated tests can reuse the compilation result instead of
rebuilding it.

Combined, these changes reduce the time spent testing runtime
during all.bash on my laptop from about 60 to about 30 seconds.
(All.bash itself runs in 5½ minutes.)

For #10571.

Change-Id: Ie2c1798b847f1a635a860d11dcdab14375319ae9
Reviewed-on: https://go-review.googlesource.com/18085

Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>