test: Run garbage collector before testing malloc numbers.

test: Run garbage collector before testing malloc numbers.

The gccgo library generates some garbage in the init routines
because it handles interfaces slightly differently.  Since the
test sets MemStats.Alloc to 0, the first time the garbage
collector runs it goes negative and the test fails.

R=rsc, r2
CC=golang-dev
https://golang.org/cl/2110044