variable_watch: Add test for watching a variable multiple times

Tests/RunCMake/variable_watch/RunCMakeTest.cmake

@@ -2,3 +2,4 @@ include(RunCMake)
 
 run_cmake(ModifiedAccess)
 run_cmake(NoWatcher)
+run_cmake(WatchTwice)

Tests/RunCMake/variable_watch/WatchTwice-stderr.txt

@@ -0,0 +1,2 @@
+From watch1
+From watch2

Tests/RunCMake/variable_watch/WatchTwice.cmake

@@ -0,0 +1,11 @@
+function (watch1)
+    message("From watch1")
+endfunction ()
+
+function (watch2)
+    message("From watch2")
+endfunction ()
+
+variable_watch(watched watch1)
+variable_watch(watched watch2)
+set(access "${watched}")