t5532: use write_script

t5532: use write_script

The recent cleanup in b7cbbff switched t5532's use of
backticks to $(). This matches our normal shell style, which
is good. But it also breaks the test on Solaris, where
/bin/sh does not understand $().

Our normal shell style assumes a modern-ish shell which
knows about $(). However, some tests create small helper
scripts and just write "#!/bin/sh" into them. These scripts
either need to go back to using backticks, or they need to
respect $SHELL_PATH. The easiest way to do the latter is to
use write_script.

While we're at it, let's also stick the script creation
inside a test_expect block (our usual style), and split the
perl snippet into its own script (to prevent quoting
madness).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>