test: doesn't use GO_GCFLAGS for tests

GO_GCFLAGS only affects the compiler that is invoked directly by the dist tool (I think). The compiler invoked by test/run.go doesn't have its command line modified by GO_GCFLAGS. So all of the tests in test/ aren't actually testing what we think they are testing on the modified builders like noopt and ssacheck.

Maybe this is unfixable as a some of the tests in test/ provide their own flags to the compiler. But it surprised me that the ssacheck checks weren't actually running on the ssacheck builder in the test directory.

Maybe there's some way to propagate GO_GCFLAGS that makes sense.

See https://go-review.googlesource.com/c/33909/ for a hack I needed to make this work for randomized value order testing.

@bradfitz @cherrymui @dr2chase