test: update ESM loader hooks to allow ambiguous format

Pull request #50314 added support for a null format value in addition to the existing commonjs or module. However, the test for ESM hooks relied on the default value of commonjs, causing it to fail when run with the --experimental-detect-module flag. This PR enables the test to detect null formats, ensuring it runs successfully with experimental detection enabled.

Sidenote: While @GeoffreyBooth and I were debugging this behavior, the single-character function arguments made it very difficult to understand, so this PR also expands them into the full names.