always clear config.log after all

delaying the logging setup until the cache use is determined (which
depends on xspec determination) causes too much trouble.

as already explained in 7ac15ab0f, there is a bit of a gray area when
exactly the log should be cleared anyway.
a more complete solution would cache the tests' output along with the
results and re-log it (with an appropriate marker) upon re-use.

Task-number: QTBUG-59565
Change-Id: I17d457598d885bceafd6505cad5ff074c4ace502
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

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