oleaut32: Add a test for BSTR cache.

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
http://testbot.winehq.org/JobDetails.pl?Key=18731

Your paranoid android.


=== W2KPROSP4 (32 bit vartype) ===
vartype.c:6166: Test failed: good_length_entries 98 out of 256

=== WXPPROSP3 (32 bit vartype) ===
vartype.c:6166: Test failed: good_length_entries 99 out of 256