Tests: Factor out RunCMake helper to get UNIX user id

Tests/RunCMake/RunCMake.cmake

@@ -323,5 +323,21 @@ function(ensure_files_match expected_file actual_file)
   endif()
 endfunction()
 
+# Get the user id on unix if possible.
+function(get_unix_uid var)
+  set("${var}" "" PARENT_SCOPE)
+  if(UNIX)
+    set(ID "id")
+    if(CMAKE_SYSTEM_NAME STREQUAL "SunOS" AND EXISTS "/usr/xpg4/bin/id")
+      set (ID "/usr/xpg4/bin/id")
+    endif()
+    execute_process(COMMAND ${ID} -u $ENV{USER} OUTPUT_VARIABLE uid ERROR_QUIET
+                    RESULT_VARIABLE status OUTPUT_STRIP_TRAILING_WHITESPACE)
+    if(status EQUAL 0)
+      set("${var}" "${uid}" PARENT_SCOPE)
+    endif()
+  endif()
+endfunction()
+
 # Protect RunCMake tests from calling environment.
 unset(ENV{MAKEFLAGS})

Tests/RunCMake/if/RunCMakeTest.cmake

@@ -4,18 +4,9 @@ run_cmake(InvalidArgument1)
 run_cmake(exists)
 if(NOT MSYS)
   # permissions and symbolic links are broken on MSYS
-  unset(uid)
-  unset(status)
-  if(UNIX)
-    set(ID "id")
-    if (CMAKE_SYSTEM_NAME STREQUAL "SunOS" AND EXISTS "/usr/xpg4/bin/id")
-      set (ID "/usr/xpg4/bin/id")
-    endif()
-    # if real user is root, tests are irrelevant
-    execute_process(COMMAND ${ID} -u $ENV{USER} OUTPUT_VARIABLE uid ERROR_QUIET
-                    RESULT_VARIABLE status OUTPUT_STRIP_TRAILING_WHITESPACE)
-  endif()
-  if(NOT status AND NOT uid STREQUAL "0")
+  # if real user is root, tests are irrelevant
+  get_unix_uid(uid)
+  if(NOT uid STREQUAL "0")
     run_cmake(FilePermissions)
   endif()
 endif()