Does not work when main script is ES module

`node script.mjs` results in `require.main` being undefined.  I'm addressing this within the next version of tap at https://github.com/tapjs/libtap/pull/21.  A bit ugly but I'm using `process.argv[1]` as long as we're not in REPL and we're not evaluating with any form of `node -p` or `node -e`.