test: do not require flags when executing a file

Instead of throwing an error in case a flag is missing, just start
a `child_process` that includes all flags. This improves the situation
for all developers in case they want to just plainly run a test.

Co-authored-by: Rich Trott <rtrott@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/26858


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Signed-off-by: Beth Griggs <Bethany.Griggs@uk.ibm.com>