Introduce tests/prof_cmocka.h

As 9f2abc75 accidentally got the ordering of some of the includes wrong,
I decided to propose my initial solution again.

Additional to that, I've opened a MR against CMocka to solve this on
their side, since I believe that the current way this is done is not
sustainable [0].

[0] https://gitlab.com/cmocka/cmocka/-/merge_requests/91

Fixes: 9f2abc75 ("Fix tests with gcc15 (uintptr_t)")
Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

CProof note: our new tests need to also be updated.

Makefile.am

@@ -124,6 +124,7 @@ unittest_sources = \
 	src/event/server_events.c src/event/server_events.h \
 	src/event/client_events.c src/event/client_events.h \
 	src/ui/tray.h src/ui/tray.c \
+	tests/prof_cmocka.h \
 	tests/unittests/xmpp/stub_vcard.c \
 	tests/unittests/xmpp/stub_avatar.c \
 	tests/unittests/xmpp/stub_ox.c \
@@ -170,6 +171,7 @@ unittest_sources = \
 	tests/unittests/unittests.c
 
 functionaltest_sources = \
+	tests/prof_cmocka.h \
 	tests/functionaltests/proftest.c tests/functionaltests/proftest.h \
 	tests/functionaltests/test_connect.c tests/functionaltests/test_connect.h \
 	tests/functionaltests/test_ping.c tests/functionaltests/test_ping.h \
@@ -284,6 +286,7 @@ endif
 
 TESTS = tests/unittests/unittests
 check_PROGRAMS = tests/unittests/unittests
+tests_unittests_unittests_CPPFLAGS = -Itests/
 tests_unittests_unittests_SOURCES = $(unittest_sources)
 tests_unittests_unittests_LDADD = -lcmocka