t: factor out FUNNYNAMES as shared lazy prereq

A fair number of tests need to check that the filesystem supports file
names including "funny" characters, like newline, tab, and double-quote.
Jonathan Nieder suggested that this be extracted into a lazy prereq in
the top-level `test-lib.sh`. This patch effects that change.

The FUNNYNAMES prereq now uniformly requires support for newlines, tabs,
and double-quotes in filenames. This very slightly decreases the power
of some tests, which might have run previously on a system that supports
(e.g.) newlines and tabs but not double-quotes, but now will not. This
seems to me like an acceptable tradeoff for consistency.

One test (`t/t9902-completion.sh`) defined FUNNYNAMES to further require
the separators \034 through \037, the test for which was implemented
using the Bash-specific $'\034' syntax. I've elected to leave this one
as is, renaming it to FUNNIERNAMES.

After this patch, `git grep 'test_\(set\|lazy\)_prereq.*FUNNYNAMES'` has
only one result.

Signed-off-by: William Chargin <wchargin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9903-bash-prompt.sh

@@ -63,18 +63,15 @@ test_expect_success 'prompt - unborn branch' '
 	test_cmp expected "$actual"
 '
 
-repo_with_newline='repo
-with
-newline'
-
-if test_have_prereq !MINGW && mkdir "$repo_with_newline" 2>/dev/null
-then
-	test_set_prereq FUNNYNAMES
-else
+if test_have_prereq !FUNNYNAMES; then
 	say 'Your filesystem does not allow newlines in filenames.'
 fi
 
 test_expect_success FUNNYNAMES 'prompt - with newline in path' '
+    repo_with_newline="repo
+with
+newline" &&
+	mkdir "$repo_with_newline" &&
 	printf " (master)" >expected &&
 	git init "$repo_with_newline" &&
 	test_when_finished "rm -rf \"$repo_with_newline\"" &&