test-lib: Infrastructure to test and check for prerequisites

Some tests can be run only if a particular prerequisite is available. For
example, some tests require that an UTF-8 locale is available. Here we
introduce functions that are used in this way:

1. Insert code that checks whether the prerequisite is available. If it is,
   call test_set_prereq with an arbitrary tag name that subsequently can be
   used to check for the prerequisite:

      case $LANG in
      *.utf-8)
            test_set_prereq UTF8
            ;;
      esac

2. In the calls to test_expect_success pass the tag name:

      test_expect_success UTF8 '...description...' '...tests...'

3. There is an auxiliary predicate that can be used anywhere to test for
   a prerequisite explicitly:

      if test_have_prereq UTF8
      then
            ...code to be skipped if prerequisite is not available...
      fi

Signed-off-by: Johannes Sixt <j6t@kdbg.org>

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