Add tests for hybrid `+' keep batching

`+' keep entries had no end-to-end test coverage. Add tests that verify:

* Multiple `+' keep entries trigger a single batched call to the
  external command, with the kept paths passed as the new pathspecs
  argument.
* The single-keep-dir case keeps going through `projectile-dir-files'
  for each subdirectory (no behavioural change there).
* `projectile--restricted-sub-projects-files' returns all submodule
  files when no subdirs are supplied, drops files outside the supplied
  subdirs when they are, and normalises subdirs without a trailing
  slash.

Also add a focused test for the new `pathspecs' argument of
`projectile-files-via-ext-command'.

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