Only search for test suite jdk files once

compatibility.mk is included 33 times on aosp, and 47 times on
internal master. Each of these would run this shell command, and
there would be an entry in the kati stamp file for each time it was
run, causing this command to be run the 33/47 times every single build.
This took ~0.2 seconds, which can be saved by only running it once.
(However these ~0.2 seconds are parallelized with other parts of the
stamp checking)

Bug: 282079550
Test: m nothing
Change-Id: I364836d1cb0cc26ca9116eda6d954170e1cb7761