test: doesn't use GO_GCFLAGS for tests

[randall77]

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

[bradfitz]

/cc @josharian

[josharian]

Maybe this is unfixable as a some of the tests in test/ provide their own flags to the compiler.

I think this might be so, in the general case. Some flags are incompatible with others in non-trivial ways (which is why cmd/go is such a bear). And the test dir generally probes the weirdest cases we can find.

I'm inclined towards adding hacks to test/run.go to solve particular problems instead, like the hack+problem in CL 33909.