tildify.el: Fix matched group indexes in end-regex building

* lisp/textmodes/tildifi.el (tildify-find-env): When looking for
a start of an ignore-environment, the regex is built by
concatenating regexes of all the environments configured in
`tildify-ignored-environments-alist'.  So for example, the following
list could be used to match TeX's \verb and \verb* commands:

    (("\\\\verb\\(.\\)" . (1))
     ("\\\\verb\\*\\(.\\)" . (1)))

This would result in the following regex being used to find the start
of any of the variants of the \verb command:

    \\\\verb\\(.\\)\\|\\\\verb\\*\\(.\\)

But now, if “\\\\verb\\*\\(.\\)” matches, the first capture group
won't match anything, and thus (match-string 1) will be nil, which
will cause building of the end-matching regex to fail.

Fix this by using capture groups from the time when the opening
regexes are matched individually.

* tests/automated/tildify-tests.el (tildify-test-find-env-group-index-bug):
New test validating fix to the above bug.

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