grep: prepare for testing binary regexes containing rx metacharacters

grep: prepare for testing binary regexes containing rx metacharacters

Add setup code needed for testing regexes that contain both binary
data and regex metacharacters.

The POSIX regcomp() function inherently can't support that, because it
takes a \0-delimited char *, but other regex engines APIs like PCRE v2
take a pattern/length pair, and are thus able to handle \0s in
patterns as well as any other character.

When kwset was imported in commit 9eceddeec6 ("Use kwset in grep",
2011-08-21) this limitation was fixed, but at the expense of
introducing the undocumented limitation that any pattern containing \0
implicitly becomes a fixed match (equivalent to -F having been
provided).

That's not something we'd like to keep in the future. The inability to
match patterns containing \0 is a leaky implementation detail.

So add tests as a first step towards changing that. In order to test
that \0-patterns can properly match as regexes the test string needs
to have some regex metacharacters in it.

There were other blind spots in the tests. The code around kwset
specially handles case-insensitive & non-ASCII data, but there were no
tests for this.

Fix all of that by amending the text being matched to contain both
regex metacharacters & non-ASCII data.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>