Basic exception guarantee testcases (redux)

[Benjamin Kosnik]

>> + // 23.2.1     required types
>> + //
>> + // A missing required type will cause a compile failure.
>> + //
>> + void
>> + requiredTypesCheck()
>> + {
>
>Very nice.

Things like this were done in a non-systematic way for the 27_io and
22_locale tests. The files that test for generic compilation check for
required types.

It would be interesting to try and do this consistently at some point
throughout the library testsuite.

-benjamin