assert: use Object.is comparison in .strictEqual

This aligns assert.strictEqual and assert.notStrictEqual with assert.deepStrictEqual to use the Object.is() comparison instead of strict equality.

It looked somewhat nicer with === but I think it makes sense to use the equality checks in both functions.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

assert