Add an option to clean the test directory

Author: dkprice

tests/runall.bat

@@ -1,3 +1,22 @@
 @ECHO off
 REM This suite requires https://github.com/inkarkat/runVimTests to run
+
+if %1.==clean. goto Clean
+if %1.==run. goto Run
+
+echo "usage: runall.bat clean|run"
+goto End
+
+
+:Clean
+echo Cleaning
+del /Q *.out 2> nul
+del /Q *.msgout 2> nul
+goto End
+
+:Run
+echo Running
 ../../runVimTests/bin/runVimTests.cmd --pure vimgrep.suite
+goto End
+
+:End