Implement Boolean debug build option debug_assert

MariaDB Server 10.3.1 introduced the compiler flag (not cmake option) DBUG_ASSERT_AS_PRINTF that converts DBUG_ASSERT in non-debug builds into printouts.

For debug builds, it would be useful to be able to convert DBUG_ASSERT into a warning or error printout, to allow execution to continue. This would allow debug builds to be used for reproducing hard failures that occur with release builds.

My proposal for the option name would be debug_assert, because we already have debug_assert_on_error and debug_assert_if_crashed_table. Typical usage could be like this:

./mtr --mysqld=--skip-debug-assert some_crashing_test