In the Windows clean script for buildbots, also clear the build dir

(so that stale test files, which can be very large, get wiped out)

Tools/buildbot/clean-amd64.bat

 @rem Used by the buildbot "clean" step.
 call "%VS90COMNTOOLS%\..\..\VC\vcvarsall.bat" x86_amd64
-cd PCbuild
 @echo Deleting .pyc/.pyo files ...
 del /s Lib\*.pyc Lib\*.pyo
+@echo Deleting test leftovers ...
+rmdir /s /q build
+cd PCbuild
 vcbuild /clean pcbuild.sln "Release|x64"
 vcbuild /clean pcbuild.sln "Debug|x64"
+cd ..

Tools/buildbot/clean.bat

@@ -2,6 +2,9 @@
 call "%VS90COMNTOOLS%vsvars32.bat"
 @echo Deleting .pyc/.pyo files ...
 del /s Lib\*.pyc Lib\*.pyo
+@echo Deleting test leftovers ...
+rmdir /s /q build
 cd PCbuild
 vcbuild /clean pcbuild.sln "Release|Win32"
 vcbuild /clean pcbuild.sln "Debug|Win32"
+cd ..