Fix variable substitution for multiple defaulted vars

The problem was that the python regex engine is strictly left-to-right,
so matching `[^}]|\\}` against \\}} meant that the `\\` was consumed by
the left of the `|`. The solution is to just switch them around.

Also add a way to run python tests from within vim, so we can actually
test this stuff.

python3/vimspector/utils.py

@@ -460,8 +460,8 @@ VAR_MATCH = re.compile(
       {(?P<braceddefault>               # or An {id:default} - default param, as
         (?P<defname>[_a-z][_a-z0-9]*)   #   an ID
         :                               #   then a colon
-        (?P<default>(?:[^}]|\})*)       #   then anything up to }, or a \}
-      )}                             |  #
+        (?P<default>(?:\\}|[^}])+)      #   then anything up to }, or a \}
+      )}                             |  #   then a }
       (?P<invalid>)                     # or Something else - invalid
     )
   """,