Supply status for missing exit codes

Following on from the work in "Handle -9 PyTest return code",
discussion with @Otto-AA and @boxed revealed it would be desirable
to handle missing return codes in full generality.

The simplest way I could think of to _do_ that, trying to eliminate
side effects, is to convert status_by_exit_code to a defaultdict,
with a default value of "suspicious" - one already defined, and
handled, elsewhere in the codebase.

Author: CyberiaResurrection

mutmut/__main__.py

@@ -67,7 +67,7 @@
 # TODO: hash of function. If hash changes, retest all mutants as mutant IDs are not stable
 
 
-status_by_exit_code = {
+status_by_exit_code = defaultdict(lambda: 'suspicious', {
     1: 'killed',
     3: 'killed',  # internal error in pytest means a kill
     -24: 'killed',
@@ -85,7 +85,7 @@
     255: 'timeout',
     -11: 'segfault',
     -9: 'segfault',
-}
+})
 
 emoji_by_status = {
     'survived': '🙁',