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.

Change-Id: I209a8fdcace94bc5cec946f5dd365d7191f44c02
Reviewed-on: https://go-review.googlesource.com/14822Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>