After the normal build and unit tests run, incrementally build
ReactorUnitTests with REACTOR_ENABLE_PRINT enabled, and run only the
unit tests that exercise the print code. This should add very little
extra time to our Kokoro builds, while ensuring we don't break the
rr::Print stuff.

Bug: b/149328074
Change-Id: I26c0db727c8edd269243f17192020a3435893c14
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41148
Presubmit-Ready: Antonio Maiorano <amaiorano@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>