test: fix async-context-frame tests with --shell

Forward execPath as test.py --shell so subprocess tests run with the
same executable as the parent test. Otherwise tests are broken if
out/Debug/node doesn't exist or is a different executble than the caller
intended.

Author: everett1992

test/parallel/test-async-context-frame.mjs

@@ -49,6 +49,7 @@ describe('AsyncContextFrame', {
     it(test, async () => {
       const proc = spawn(python, [
         testRunner,
+        '--shell', process.execPath,
         '--node-args=--experimental-async-context-frame',
         test,
       ], {