Set Makefile default goal to `tests` instead of `clean`

By virtue of being the first goal in the Makefile, `clean` is currently
the default. Personally I feel this is slightly aggressive (having
accidentally wiped out scratch files twice now due to bare `make`s) so
this sets `.DEFAULT_GOAL` to `tests`, which is non-destructive and
(based on no data other than my own usage) probably more desired.