Support graceful termination via CTRL+C

[moodmosaic]

Currently CTRL+C doesn't cleanly terminate test sessions. Should:

• Stop any running test immediately
• Print results of completed tests
• Show partial coverage data if available
• Clean up any temporary state

Example:

Starting invariant testing type for the counter contract...

₿        1 Ӿ        9   wallet_4        counter decrement 
₿        1 Ӿ        9   wallet_4 [PASS] counter invariant-counter-gt-zero 
₿      835 Ӿ      845   wallet_7        counter add 6
₿      835 Ӿ      845   wallet_1 [PASS] counter invariant-counter-gt-zero 
^C
Terminating...
Tests:      4 passed, 2 discarded, 94 skipped
Time:       1.24s
Coverage:   7.5%

This requires #85 as prerequisite.