Compile once when running tests, rather than recompiling for each timezone

[danielcompton]

With #134, we run tests under many timezones now by exporting the TZ variable. However we create a new Boot process for each set of timezone tests and recompile ClojureScript twice (:simple and :advanced). It should be possible to compile once for :simple, then run the test suite for all the different timezones, then recompile again for :advanced and run the test suite again.

This might mean moving away from Doo as it ties the compilation to the running of the tests.