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 #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/bitcoin#30975 (comment)

Co-authored-by: Hennadii Stepanov <[email protected]>

Author: ryanofsky

CMakeLists.txt

@@ -94,7 +94,7 @@ option(BUILD_DAEMON "Build bitcoind executable." ON)
 option(BUILD_GUI "Build bitcoin-qt executable." OFF)
 option(BUILD_CLI "Build bitcoin-cli executable." ON)
 
-option(BUILD_TESTS "Build test_bitcoin executable." ON)
+option(BUILD_TESTS "Build test_bitcoin and other unit test executables." ON)
 option(BUILD_TX "Build bitcoin-tx executable." ${BUILD_TESTS})
 option(BUILD_UTIL "Build bitcoin-util executable." ${BUILD_TESTS})
 

cmake/libmultiprocess.cmake

@@ -25,4 +25,8 @@ function(add_libmultiprocess subdir)
   mark_as_advanced(CapnProto_kj_IMPORTED_LOCATION)
   mark_as_advanced(CapnProto_kj-test_IMPORTED_LOCATION)
   mark_as_advanced(CapnProto_kj-tls_IMPORTED_LOCATION)
+  if(BUILD_TESTS)
+    # Add tests to "all" target so ctest can run them
+    set_target_properties(mptests PROPERTIES EXCLUDE_FROM_ALL OFF)
+  endif()
 endfunction()