Before this commit:

```
$ zig test lib/std/fs/test.zig --main-pkg-path lib/std --zig-lib-dir lib
2170 passed; 37 skipped; 0 failed.
```

After this commit:

```
$ zig test lib/std/fs/test.zig --main-pkg-path lib/std --zig-lib-dir lib
All 45 tests passed.
```

This matches stage1 behavior:

```
$ zig test -fstage1 lib/std/fs/test.zig --main-pkg-path lib/std --zig-lib-dir lib
All 45 tests passed.
```

All tests are still run if `zig test` is run directly on `lib/std/std.zig`:

```
$ zig test lib/std/std.zig --main-pkg-path lib/std --zig-lib-dir lib
2170 passed; 37 skipped; 0 failed.
```

`zig build test-std` is unaffected by this change.

Closes #12926