cmd/compile: unify compilation of compiler tests

Before this CL we would build&run each test file individually.
Building the test takes most of the time, a significant fraction of a
second. Running the tests are really fast.

After this CL, we build all the tests at once, then run each
individually. We only have to run the compiler&linker once (or twice,
for softfloat architectures) instead of once per test.

While we're here, organize these tests to fit a bit more into the
standard testing framework.

This is just the organizational CL that changes the testing framework
and migrates 2 tests.  Future tests will follow.

R=go1.12

Update #26469

Change-Id: I1a1e7338c054b51f0c1c4c539d48d3d046b08b7d
Reviewed-on: https://go-review.googlesource.com/126995


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>