logcat: fix tests broken from new formatting

1) `logcat -g` now contains 'readable', so fix sscanf() appropriately
2) Remove tests that the security buffer isn't returned as an error,
since it's not important to hide this buffer from the logging front
end, and more importantly: the expected failures no longer happen.
3) Update invalid_buffer for the new error format.

Test: logcat-unit-tests
Change-Id: I1a607815bdc7b1135e9a0e97f8b7334f2d259e31