v6.x backport - test: convert var->const/let in tests

I'll fix up the commit messages once the tests are passing. EDIT: They are.

Backport of #10685, I couldn't cherry-pick as it brought in hundreds of unrelated changes, so I redid the eslint --fix and manual fixup for v6.x.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test, tools