Fix wonkiness when mutant_names tuple includes at least one space

Second step, _fix_ the wonkiness and _verify_ the fix.

Author: CyberiaResurrection

mutmut/__main__.py

@@ -962,10 +962,13 @@ def _run(mutant_names: Union[tuple, list], max_children: Union[None, int]):
     # TODO: we should be able to get information on which tests killed mutants, which means we can get a list of tests and how many mutants each test kills. Those that kill zero mutants are redundant!
     os.environ['MUTANT_UNDER_TEST'] = 'mutant_generation'
     ensure_config_loaded()
-    if isinstance(mutant_names, tuple) and 1 == len(mutant_names):
-        bits = mutant_names[0].split(' ')
-        if 1 < len(bits):
-            mutant_names = bits
+    if isinstance(mutant_names, tuple):
+        bits = []
+        for item in mutant_names:
+            bits.extend(item.split(' '))
+
+        if len(bits) > len(mutant_names):
+            mutant_names = tuple(bits)
 
     if max_children is None:
         max_children = os.cpu_count() or 4