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.

tests/python/Test_ExpandReferencesInDict.py

@@ -0,0 +1,79 @@
+import sys
+import unittest
+from unittest.mock import patch
+from vimspector import utils
+
+
+class TestExpandReferencesInDict( unittest.TestCase ):
+  def test_ExpandReferencesInDict( self ):
+    mapping = {
+      'one': 'one',
+      'two': 'TWO',
+      'bool': True,
+      'words': 'these are some words'
+    }
+    calculus = {
+      'three': lambda : 1 + 2
+    }
+    CHOICES = {
+      'five': '5ive!'
+    }
+
+    def AskForInput( prompt, default_value = None ):
+      if default_value is not None:
+        return default_value
+
+      return 'typed text'
+
+    d = {
+      'dollar': '$$',
+      'not_a_var': '$${test}',
+      'one': '${one}',
+      'two': '${one} and ${two}',
+      'three': '${three}',
+      'four': '${four}',
+      'five': '${five}',
+      'list': [ '*${words}' ],
+      'list1': [ 'start', '*${words}', 'end' ],
+      'list2': [ '*${words}', '${three}' ],
+      'list3': [ '${one}', '*${words}', 'three' ],
+      'dict#json': '{ "key": "value" }',
+      'bool#json': 'false',
+      'one_default': '${one_default:one}',
+      'two_default': '${two_default_1:one} and ${two_default_2:two}',
+      'one_default2': '${one_default2:${one\\}}',
+      'two_default2':
+        '${two_default2_1:${one\\}} and ${two_default2_2:${two\\}}',
+    }
+
+    e = {
+      'dollar': '$',
+      'not_a_var': '${test}',
+      'one': 'one',
+      'two': 'one and TWO',
+      'three': '3',
+      'four': 'typed text',
+      'five': '5ive!',
+      'list': [ 'these', 'are', 'some', 'words' ],
+      'list1': [ 'start', 'these', 'are', 'some', 'words', 'end' ],
+      'list2': [ 'these', 'are', 'some', 'words', '3' ],
+      'list3': [ 'one', 'these', 'are', 'some', 'words', 'three' ],
+      'dict': {
+        'key': 'value',
+      },
+      'bool': False,
+      'one_default': 'one',
+      'two_default': 'one and two',
+      'one_default2': 'one',
+      'two_default2': 'one and TWO',
+    }
+
+    with patch( 'vimspector.utils.AskForInput', side_effect = AskForInput ):
+      utils.ExpandReferencesInDict( d, mapping, calculus, CHOICES )
+
+    self.assertDictEqual( d, e )
+
+
+unittest.main( module=__name__,
+               testRunner=unittest.TextTestRunner( sys.stdout ),
+               exit=False )