test defs: new functions to analyze configure help screen

test defs: new functions to analyze configure help screen

* tests/defs (extract_configure_help, grep_configure_help): New
functions.
* tests/maintmode-configure-msg.test: Use them, reducing code
duplication and test brittleness.
* tests/help-depend.test: Likewise.
* tests/help-depend2.test: Likewise.
* tests/help-dmalloc.test: Likewise.
* tests/help-lispdir.test: Likewise.
* tests/help-multilib.test: Likewise.
* tests/help-python.test: Likewise.
* tests/help-regex.test: Likewise.
* tests/help-silent.test: Likewise.
* tests/help-upc.test: Likewise.
* tests/help-init.test: Make grepping of configure help screen
slightly stricter.
* tests/self-check-configure-help.test: New self test.
* tests/Makefile.am (TESTS): Add it.

From a report by Jim Meyering.