compat: libraries can't spawn Deno subprocess properly

[bartlomieju]

Hit this problem when trying to get tap to work in "--compat" mode.

This snippet shows how tap runs a single file with tests:

        debug('js file', file)
        /* istanbul ignore next - version specific behavior */
        const experimental = /^v10\./.test(process.version) && /\.mjs$/.test(file)
          ? ['--experimental-modules'] : []

        const args = [
          ...options['node-arg'],
          ...experimental,
          file,
          ...options['test-arg']
        ]
        tap.spawn(node, args, opt, file)

node is defined as:

const node = process.execPath

Later it constructs a command to be run using something like this:

const cp = require("child_process");
const p = {
  spawn: async (name, ...rest) => cp.spawn(...rest);
};
await p.spawn(this.name, this.command, this.args, options);

tap is not aware that it must run the process by using deno run --compat --unstable args and only uses deno.