test: fix error handling test-http-full-response

The way it is currently written, test-http-full-response will fail if
there is a problem with spawning that doesn't include `ab` or `api` in
`stderr`, but it will fail with a misleading mismatched-calls
`common.mustCall()` error.

Alter the error handling so that it rethrows the actual error, providing
better information.

PR-URL: #14252
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-http-full-response.js

@@ -25,8 +25,7 @@ function runAb(opts, callback) {
         common.printSkipMessage(`problem spawning \`ab\`.\n${stderr}`);
         process.reallyExit(0);
       }
-      process.exit();
-      return;
+      throw err;
     }
 
     let m = /Document Length:\s*(\d+) bytes/i.exec(stdout);