tests: use 'test_atexit' to stop httpd

Use 'test_atexit' to run cleanup commands to stop httpd at the end of
the test script or upon interrupt or failure, as it is shorter,
simpler, and more robust than registering such cleanup commands in the
trap on EXIT in the test scripts.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5551-http-fetch-smart.sh

@@ -434,5 +434,4 @@ test_expect_success 'server-side error detected' '
 	grep "server-side error" actual
 '
 
-stop_httpd
 test_done