test: refactor test-child-process-exec-error

Checklist

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

Affected core subsystem(s)

test child_process

Description of change

• assert.equal() -> assert.strictEqual()
• regex -> .include()
• Change variable representing a function from fun to idiomatic fn
• var -> const