CI: also run go test -race
CI: also run go test -race

Separately from the other "go test" run, for various reasons:

* It can be slow, so no need to run it in all Go versions
* If a test fails only when in race mode, that's useful info
* No need to run the race detector if the tests failed without it

Fixes #363.

mvdan/tyk,mvdan/tyk,mvdan/tyk,mvdan/tyk,lonelycode/tyk,mvdan/tyk,mvdan/tyk,lonelycode/tyk,mvdan/tyk,lonelycode/tyk,mvdan/tyk