Right now, we can only run the e2e gio app with -race, not without
-race, because the flag is hard-coded in the tests.
The reason for this change was that 'GOFLAGS=-race go test' would fail
with the JS test, since js/wasm doesn't support the race detector. Fix
that by skipping the JS test when -race is used.
Now, we can run multiple levels of -race:
go test # no -race at all
go test -race # -race for the tests, not the e2e gio app
GOFLAGS=-race go test # -race for everything (best-effort)
To detect the race detector being on, we use a file with a build tag.
Signed-off-by: Daniel Martí <mvdan@mvdan.cc>