test: clean up `test-child-process-exec-cwd.js`

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test

Description of change

This patch provides minor fixes to test-child-process-exec-cwd.js. Notably:

• Changed == to ===.
• Changed var to const where possible.