(docs/fix): remove erroneous Test README sub-header

- it said "two" when there were only two, but there are more now
- the fixture READMEs don't have sub-headers so just remove it entirely
  to be consistent

test/README.md

@@ -1,7 +1,5 @@
 # Tests
 
-There are two main directories here:
-
 - `unit` contains unit tests of internals
 - `e2e` contains end-to-end (E2E) tests of the CLI
 - `integration` contains tests ensuring that common or recommended plugins work properly together with TSDX