tests,meson: add test-bus-vtable, compiled as C and C++

This test is mostly a compilation test that checks that various defines in
sd-bus-vtable.h are valid C++. The code is executed, but the results are not
checked (apart from sd-bus functions not returning an error). test-bus-objects
contains pretty extensive tests for this functionality.

The C++ version is only added to meson, since it's simpler there.
Because of the .cc extension, meson will compile the executable with c++.

This test is necessary to properly check the macros in sd-bus-vtable.h. Just
running the headers through g++ is not enough, because the macros are not
exercised.

Follow-up for #5941.

.gitignore

@@ -160,6 +160,8 @@
 /test-bus-server
 /test-bus-signature
 /test-bus-track
+/test-bus-vtable
+/test-bus-vtable-cc
 /test-bus-zero-copy
 /test-calendarspec
 /test-cap-list