test: fix common.PIPE path bug

`common.PIPE` is returning a path name in `test` rather than in the
`tmp` directory for each test. This is causing multiple test failures in
CI. Make the path name inside the temporary directories again. This way
the pipe is removed by `common.refreshTmpDir()` on POSIX.

The bug in `common.PIPE` was introduced in
c34ae480.

PR-URL: https://github.com/nodejs/node/pull/16364
Fixes: https://github.com/nodejs/node/issues/16290
Fixes: https://github.com/nodejs/node/issues/16323


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>