Tests/UnityBuild: add shortened path tests

Tests/RunCMake/UnityBuild/RunCMakeTest.cmake

@@ -71,3 +71,7 @@ endfunction()
 
 run_test(unitybuild_runtest)
 run_test(unitybuild_object_library)
+
+if (CMAKE_GENERATOR MATCHES "NONE")
+  run_build(unitybuild_cxx_short)
+endif ()

Tests/RunCMake/UnityBuild/unitybuild_cxx_short-check.cmake

@@ -0,0 +1,5 @@
+set(unitybuild_cxx "${RunCMake_TEST_BINARY_DIR}/.o/9bc0d702/Unity/unity_0_cxx.cxx")
+if(NOT EXISTS "${unitybuild_cxx}")
+  set(RunCMake_TEST_FAILED "Generated unity source files ${unitybuild_cxx} does not exist.")
+  return()
+endif()

Tests/RunCMake/UnityBuild/unitybuild_cxx_short.cmake

@@ -0,0 +1,14 @@
+set(CMAKE_INTERMEDIATE_DIR_STRATEGY SHORT CACHE STRING "" FORCE)
+
+project(unitybuild_cxx_short CXX)
+
+set(srcs "")
+foreach(s RANGE 1 8)
+  set(src "${CMAKE_CURRENT_BINARY_DIR}/s${s}.cxx")
+  file(WRITE "${src}" "int s${s}(void) { return 0; }\n")
+  list(APPEND srcs "${src}")
+endforeach()
+
+add_library(tgt SHARED ${srcs})
+
+set_target_properties(tgt PROPERTIES UNITY_BUILD ON)