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>