tests: reinstate warnings-as-errors on clang+windows

Author: jll63

test/Jamfile

@@ -23,10 +23,9 @@ project
 
     <warnings>extra
 
-    <toolset>msvc:<warnings-as-errors>on
+    <toolset>clang:<warnings-as-errors>on
     <toolset>gcc:<warnings-as-errors>on
-    <toolset>clang,<target-os>linux:<warnings-as-errors>on
-    <toolset>clang,<target-os>darwin:<warnings-as-errors>on
+    <toolset>msvc:<warnings-as-errors>on
   ;
 
 alias unit_test_framework