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/lib/run_test.vim

@@ -82,6 +82,16 @@ func! Abort( timer_id )
   qa!
 endfunc
 
+func! TestLog( msg )
+  if type( a:msg ) == v:t_string
+    let msg = [ a:msg ]
+  else
+    let msg = a:msg
+  endif
+
+  call extend( s:messages, msg )
+endfunc
+
 func RunTheTest(test)
   echo 'Executing ' . a:test
 
@@ -152,8 +162,6 @@ func RunTheTest(test)
     augroup END
 
     exe 'call ' . a:test
-
-    au! EarlyExit
   catch /^\cskipped/
     call add(s:messages, '    Skipped')
     call add(s:skipped,
@@ -193,6 +201,8 @@ func RunTheTest(test)
     call s:TestFailed()
   endtry
 
+  au! EarlyExit
+
   call timer_stop( timer )
 
   " In case 'insertmode' was set and something went wrong, make sure it is