Race condition running test suite with -jX

[brson]

As reported in #3962, running make check -j8 can result in random failures in building aux files, probably because of this line:

https://github.com/mozilla/rust/blob/incoming/mk/tests.mk#L95

These dependencies don't guarantee that the two 'clean' rules run before the tests.

[catamorphism]

I haven't seen this lately, and 95 is now a blank line. Closing; reopen if you can reproduce.