t5564: fix test hang under zsh's sh mode

t5564: fix test hang under zsh's sh mode

This test starts a SOCKS server in Perl in the background and then kills
it after the tests are done.  However, when using zsh (in sh mode) in
the tests, the start_socks function hangs until the background process
is killed.

Note that this does not reproduce in a simple shell script, so there is
likely some interaction between job handling, our heavy use of eval in
the test framework, and possibly other complexities of our test
framework.  What is clear, however, is that switching from a compound
statement to a subshell fixes the problem entirely and the test passes
with no problem, so do that.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>