add wml_unit_tests test aggregator script

This bash script runs a sequence of WML unit test scenarios.
If all pass the result is 0.
If any fails, the result is 1, and the std::cerr stored in "error.log".
We also add error.log to the .gitignore list.

It shows how --timeout and --validcache may be used in such a script

.gitignore

@@ -72,3 +72,4 @@ projectfiles/VC1*
 projectfiles/VC201*
 *Neuer Ordner*
 Thumbs.db
+error.log

wml_unit_tests

@@ -0,0 +1,7 @@
+#!/bin/bash
+set -e
+set -o pipefail
+./wesnoth -u unit_test_1 2> error.log
+./wesnoth --validcache -u unit_test_2 2> error.log
+./wesnoth --validcache -u unit_test_3 2> error.log
+rm error.log