re-enabling some tests to check for usability.

[SVN r32406]

status/explicit-failures-markup.xml

@@ -1406,7 +1406,7 @@
             <toolset name="sunpro-5_3-sunos"/>
             <note author="Douglas Gregor" refid="3"/>
         </mark-unusable>
-        <mark-unusable>
+        <!-- RG: testing usability <mark-unusable>
             <toolset name="gcc-2.95.3*"/>
             <toolset name="gcc-2.95.3-linux"/>
             <toolset name="gcc-2.95.3-stlport-4.6.2-linux"/>
@@ -1416,7 +1416,7 @@
 		  support enable_if. 
                 </p>
             </note>
-        </mark-unusable>
+        </mark-unusable> -->
         <test name="constructors">
             <mark-failure>
                <toolset name="msvc"/>