cmd/dist: actually only compile tests with -compile-only
Currently, "dist test -compile-only" still runs the test binaries,
just with -run=^$ so no tests are run. It does this because, until
recently, "go test -c" would fail if passed multiple test packages.
But this has some unexpected consequences: init code still runs,
TestMain still runs, and we generally can't test cross-compiling of
tests.
Now that #15513 is fixed, we can pass multiple packages to "go test
-c". Hence, this CL make dist just use "go test -c" as one would
expect.
Found in the course of working on #37486, though it doesn't really
affect that.