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).