util/log: make TestLogScope useful even during panic().
util/log: make TestLogScope useful even during panic().

Prior to this patch, TestLogScope would only use t.Failed() to
determine whether to keep the log files. This is unforunate in case of
a panic, which is an error case but may occur before any call to
t.Fatal/t.Error -- in other words a test may be panicking but have
t.Failed() false.

This patch corrects the situation by ensuring the log files are kept
also when a panic occurs.

Moreover, the message "test log files left over" is now reported on
standard error rather than t.Error(), because during a panic the
results of a testing.T may not be printed at all.

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