check-files.py: clean up class structure
check-files.py: clean up class structure

Line issue trackers are conceptually a subclass of file issue
trackers: they're file issue trackers where issues arise from checking
each line independently. So make it an actual subclass.

Pylint pointed out the design smell: there was an abstract method that
wasn't always overridden in concrete child classes.

NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls