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/utils.test.vim

@@ -0,0 +1,28 @@
+function! SetUp()
+  call vimspector#test#setup#SetUpWithMappings( v:none )
+  py3 import vim
+  py3 __import__( 'vimspector' )
+endfunction
+
+function! ClearDown()
+  call vimspector#test#setup#ClearDown()
+endfunction
+
+function! s:RunPyFile( file_name )
+  redir => py_output
+  try
+    let v:errmsg = ''
+    silent! execute 'py3file python/' .. a:file_name
+  finally
+    redir END
+    call TestLog( [ a:file_name .. ' output:' ] + split( py_output, '\n' ) )
+  endtry
+
+  if v:errmsg !=# ''
+    call assert_report( v:errmsg )
+  endif
+endfunction
+
+function! Test_ExpandReferencesInDict()
+  call s:RunPyFile( 'Test_ExpandReferencesInDict.py' )
+endfunction