Fix run_tests.bat [ci skip]

Author: Abscissa

run_tests.bat

@@ -1,7 +1,7 @@
 @echo off
 
 rem Compile using $DMD if it exists, otherwise use dmd
-if not "%DMD%" == "" set DMD=dmd
+if "%DMD%" == "" set DMD=dmd
 
 echo DMD=%DMD%
 %DMD% -ofrun_tests_bin run_tests.d && run_tests_bin %*