tests: Bluetooth: Mesh: Add ability to abort bsim mesh tests by ctrl-c

Sometimes there is a need to interrupt mesh babblesim tests when
debugging or implementing new tests. Calling Ctrl-C doesn't work as it
only aborts the test script, but not the executables. This fix adds a
trap for Ctrl-C to kill all test instances run by the script.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>