Fix ESLint config

[fisker]

Due to ESLint ignore behavior change, we can't correctly lint files in tests/format/ except jsfmt.spec.js. eslint/eslint#17964 (comment)

We need update

prettier/.eslintignore

Lines 4 to 5 in 6c6a332

	tests/format/**/*.*
	!tests/format/**/jsfmt.spec.js

to

# Ignore directories and files in `tests/format`
tests/format/**/*
# Unignore directories and `jsfmt.spec.js` file
!tests/format/**/
!tests/format/**/jsfmt.spec.js

To verify it works as expected,

• Update tests/format/flow-repo/esproposal_class_instance_fields.warn/jsfmt.spec.js to use run_spec(__dirname, ["flow"])
• Run yarn eslint tests/format/flow-repo/esproposal_class_instance_fields.warn/, should report error for jsfmt.spec.js but not other files.

[fisker]

Fixed by #16208