Merge branch 'mm/test-grep-lint'

Merge branch 'mm/test-grep-lint'

The test suite has been updated to use the 'test_grep' helper instead
of bare 'grep' for test assertions, allowing file contents to be
printed on failure for easier debugging.  A new 'greplint' linter has
been introduced to detect and prevent new bare 'grep' assertions from
being added to the test suite.

* mm/test-grep-lint:
  t: add greplint to detect bare grep assertions
  t: convert grep assertions to test_grep
  t: fix Lexer line count for $() inside double-quoted strings
  t: extract chainlint's parser into shared module
  t: fix grep assertions missing file arguments
  t/README: document test_grep helper