Make sure .txt files are not identified as crashed test
This is a regression from !3867 (merged).
Previously .txt files with full backtrace may be identified as a crashed test:
I:Core dumps were found for the following system tests: I: core.19948-backtrace.txt I: shutdown
Now .txt files are removed from the list.
This is not really useful for
bin/tests/system/testsummary.sh is triggered only on Windows, but
testsummary.sh is still used on