Some variable values were missing and perl constructs failed.

Initialized the variables and refactored the gcov functions.


.bzrignore:
  Bug#40446 - mysql-test-run --gcov is broken
  Added gcov log files.
mysql-test/lib/mtr_gcov.pl:
  Bug#40446 - mysql-test-run --gcov is broken
  Refactored the gcov functions.
mysql-test/mysql-test-run.pl:
  Bug#40446 - mysql-test-run --gcov is broken
  Initialized gcov variables.
  Added usage information.