test: minimize impact of native compilation.

test: minimize impact of native compilation.

Native compilation is kindof useless in the test suite because we
throw away the cache after every subtest.  The test suite could in
principle share an eln cache within a given test file; for now try to
minimize the amount of native-compilation. There is an intermittent
bug where emacs loses track of its default-directory; I suspect (but
have no proof) that bug is related to native compilation and/or race
conditions. This patch seems to prevent that bug (or at least reduce
its frequency).