C++: Tests: Reset diagnostic messages

...otherwise they add up and subsequent tests checking for the error count will
fail:

FAIL!  : tst_AST::objc_try_statement_1() Compared values are not the same
   Actual   (diag.errorCount): 5
   Expected (0): 0
   Loc: [/home/nikolai/dev/creator/qtcreator-master/tests/auto/cplusplus/ast/tst_ast.cpp(1400)]
...

Change-Id: I8b4356331e95dceb1e99bb26e6ebf4591d70a08c
Reviewed-by: David Schulz <david.schulz@digia.com>