[msi/tests] Prevent crash when ConvertSidToStringSidA is not available

[Paul Vriens]

Hi,

Previous logic didn't quite work as pConvertSidToStringSidA would have
been unconditionally called on the second invocation of get_user_sid().
This means that these tests would crash on NT4 and below.

I also got rid of the trace when a function is not exported. We are
testing and reporting them already with a win_skip().

Changelog
   Prevent crash when ConvertSidToStringSidA is not available

-- 
Cheers,

Paul.

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