tests: fix main symbol duplication

So far I got these errors before patching:

libtool: link: cc -o .libs/headers-test -pthread -Wall -Wextra -Wno-unused-parameter -g -Wstrict-prototypes -Wmissing-prototypes -fvisibility=hidden -O2 -pipe tests/headers-test.o tests/headers-protocol-test.o tests/headers-protocol-core-test.o /tmp/obj/wayland-1.16.0/build-amd64/.libs/libtest-runner.a -L.libs -lwayland-client -lffi -lm -lwayland-server -lkvm -Wl,-rpath-link,/usr/local/lib
ld: error: duplicate symbol: main
>>> defined at headers-test.c:53 (/tmp/obj/wayland-1.16.0/wayland-1.16.0/tests/headers-test.c:53)
>>>            tests/headers-test.o:(main)
>>> defined at test-runner.c:377 (/tmp/obj/wayland-1.16.0/wayland-1.16.0/tests/test-runner.c:377)
>>>            test-runner.o:(.text+0x250) in archive /tmp/obj/wayland-1.16.0/build-amd64/.libs/libtest-runner.a

libtool: link: cc -o .libs/exec-fd-leak-checker -pthread -Wall -Wextra -Wno-unused-parameter -g -Wstrict-prototypes -Wmissing-prototypes -fvisibility=hidden -O2 -pipe tests/exec-fd-leak-checker.o /tmp/obj/wayland-1.16.0/build-amd64/.libs/libtest-runner.a -L.libs -lwayland-client -lffi -lm -lwayland-server -lkvm -Wl,-rpath-link,/usr/local/lib
ld: error: duplicate symbol: main
>>> defined at exec-fd-leak-checker.c:57 (/tmp/obj/wayland-1.16.0/wayland-1.16.0/tests/exec-fd-leak-checker.c:57)
>>>            tests/exec-fd-leak-checker.o:(main)
>>> defined at test-runner.c:377 (/tmp/obj/wayland-1.16.0/wayland-1.16.0/tests/test-runner.c:377)
>>>            test-runner.o:(.text+0x250) in archive /tmp/obj/wayland-1.16.0/build-amd64/.libs/libtest-runner.a

Makefile.am: error: object 'tests/test-helpers.$(OBJEXT)' created both with libtool and without

libtool: link: cc -o .libs/fixed-benchmark -pthread -Wall -Wextra -Wno-unused-parameter -g -Wstrict-prototypes -Wmissing-prototypes -fvisibility=hidden -O2 -pipe tests/fixed-benchmark.o /tmp/obj/wayland-1.16.0/build-amd64/.libs/libtest-runner.a -L.libs -lwayland-client -lffi -lm -lwayland-server -lkvm -Wl,-rpath-link,/usr/local/lib
ld: error: duplicate symbol: main
>>> defined at fixed-benchmark.c:100 (/tmp/obj/wayland-1.16.0/wayland-1.16.0/tests/fixed-benchmark.c:100)
>>>            tests/fixed-benchmark.o:(main)
>>> defined at test-runner.c:377 (/tmp/obj/wayland-1.16.0/wayland-1.16.0/tests/test-runner.c:377)
>>>            test-runner.o:(.text+0x250) in archive /tmp/obj/wayland-1.16.0/build-amd64/.libs/libtest-runner.a

This commit fixes all of that.

Signed-off-by: Leonid Bobrov <mazocomp@disroot.org>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>