Remove an apparently obsolete TODO comment.

Looks like 83cda1567fe3be4ec91bd71577ef1f85c5ed6070 should have
removed this. At any rate, it appears to me that all test flags are
used.
Handle build tags by adding -tags flag.

Obviously, this means we have to pass -tags down to "go test".
Less obviously, we also have to pass the tags to go/build so it finds
files enabled by custom tags. This required creating a custom build
context instead of using the default.