Recent changes caused vet to build the binary for each Test function.
This is wasteful and will become only more so as more tests are added.
Use testing.Main to build only once.
Verified that compilation errors still appear if the binary cannot be
built.
Before:
real 0m11.169s
user 0m18.328s
sys 0m2.152s
After:
real 0m5.132s
user 0m9.404s
sys 0m1.168s
Of course if the compiler were fast we might not notice, but vet is
a big program and growing bigger all the time, as are the tests.