replace ==, !=, some instances of var, and assert.equal()

[jennabelle]

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

tests

Description of change

• replaced == with ===
• replaced != with !==
• replaced some var instances with const
• replaced assert.equal() with assert.strictEqual()

[MylesBorins]

@jennabelle thanks so much for this PR! It would appear that you are going to need to rebase against master as it would appear there is a conflict in test/parallel/test-file-write-stream2.js

[jennabelle]

Sorry i messed up. Had to re-do everything --> new pull request #8770 has no base conflicts