Filter unprintable chars out of all test output.

Filter unprintable chars out of all test output.

Previously, unprintable characters were filtered out of test output
while the output strings were being formatted by either qt_snprintf() or
qt_asprintf().  Any strings not formatted by one of those functions
weren't filtered at all, and any strings passed more than once would be
filtered more than once.

This commit separates the filtering of output strings from their
formatting, leaving the filtering until just before the strings are
written to the output stream.  For now, the filtering is done by a
protected method of QAbstractTestLogger, but this could easily be
changed to a virtual method in future to allow different filtering
for loggers with different output character sets.

Change-Id: Ia4bb49cd10d37c84af75d2cf58325d27f0e16d99
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>