In -race mode, the dist test command only registers the std, race,
osusergo, and amd64ios tests before returning early from
(*tester).registerTests. Prior to CL 450018, the osusergo and amd64ios
tests weren't even affected by -race mode, so it seems their inclusion
was unintentional. CL 450018 lifted the logic to run tests in race
mode, which means these tests went from running without -race to
running with -race. Unfortunately, amd64ios is not compatible with
-race, so it is now failing on the darwin-amd64-race builder.
Fix this by omitting the osusergo and amd64ios tests from -race mode,
since it seems like they were really intended to be included anyway.