t5541: don't call start_httpd after sourcing lib-terminal.sh

Since 83d842dc8 "make test" using prove fails for some setups in t5541
with:

   "Parse errors: No plan found in TAP output"

Running t5541 on its own fails with:

   "error: Can't use skip_all after running some tests"

This happens because "start_httpd" (which determines if the test is to
be skipped) is called after sourcing lib-terminal.sh (which sets up the
terminal using test_expect_success).

Fix that by calling "start_httpd" before sourcing lib-terminal.sh.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>