chore: added .only for karma tests

[jhefferman-sfdc]

Details

Does this pull request introduce a breaking change?

• 😮‍💨 No, it does not introduce a breaking change.

Does this pull request introduce an observable change?

• 🤞 No, it does not introduce an observable change.

[wjhsf]

Can this logic be shared with the other karma .only checks?

lwc/packages/@lwc/integration-karma/scripts/karma-plugins/hydration-tests.js

Lines 209 to 210 in 5d0c085

	// You can add an `.only` file alongside an `index.spec.js` file to make it `fdescribe()`
	const onlyFileExists = await existsUp(suiteDir, '.only');

lwc/packages/@lwc/integration-karma/scripts/karma-configs/hydration/base.js

Lines 48 to 49 in 5d0c085

	// check if a .only file exists
	const onlyFile = globSync('**/*/.only', { cwd: BASE_DIR, absolute: true });