Separate the C/POSIX locale test from the rest; make it more thorough
and more correct. This fixes a problem reported by martin@ when the
test is compiled with -funsigned-char.

Add ISO10646 versions of these tests, conditional on __STDC_ISO_10646__ .
Also make the tests a bit more verbose, to aid debugging when they fail.

Add tests for btowc(3)/wctob(3) and enable compilation of the test for
digittoint(3).

The digittoint(3) test is skipped since we don't provide that function yet.

One of the test cases for btowc(3)

Add tests for btowc(3)/wctob(3) and enable compilation of the test for
digittoint(3).

The digittoint(3) test is skipped since we don't provide that function yet.

One of the test cases for btowc(3) is also skipped, since it tests conversion
to Unicode---whereas our wchar_t representation is locale-dependent.

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