cmake: Fix ctest mptest "Unable to find executable" errors

This change is technically not needed to add libmultiprocess as a subtree, but
it avoids a CI failure in followup PR bitcoin#30975 which enables multiprocess build
option in more CI jobs. In that PR, several jobs fail due to the mptest
executable not being built by default, as reported
bitcoin#30975 (comment)

Author: ryanofsky

Diff for: cmake/libmultiprocess.cmake

@@ -31,4 +31,6 @@ function(add_libmultiprocess subdir)
   # build rules can find the libmultiprocess include directory and avoid
   # "error: Import failed: /mp/proxy.capnp" errors from capnp.
   set(MP_INCLUDE_DIR "${MP_INCLUDE_DIR}" PARENT_SCOPE)
+  # Add tests to "all" target so ctest can run them
+  set_target_properties(mptests PROPERTIES EXCLUDE_FROM_ALL OFF)
 endfunction()