Improve .prettierignore file (#17632)

Co-authored-by: fisker <lionkay@gmail.com>

.prettierignore

@@ -1,6 +1,9 @@
 **/.yarn/*
 
+# Ignore directories and files in 'tests/format'
 /tests/format/**/*.*
+# Unignore directories and 'format.test.js' file
+!/tests/format/**/
 !/tests/format/**/format.test.js
 /tests/integration/cli/
 /tests/integration/plugins/**/*.*