Tests: enable Swift PM tests in Windows toolchain build

Enable the SwiftPM tests in the WIndows toolchain build to get extra
confidence the change did not introduce any regression on the Windows
platform.

Author: bkhouri

utils/build-windows-toolchain.bat

@@ -60,7 +60,7 @@ set TMPDIR=%BuildRoot%\tmp
 set NINJA_STATUS=[%%f/%%t][%%p][%%es] 
 
 :: Build the -Test argument, if any, by subtracting skipped tests
-set TestArg=-Test lld,lldb,swift,dispatch,foundation,xctest,swift-format,sourcekit-lsp,
+set TestArg=-Test lld,lldb,swift,dispatch,foundation,xctest,swift-format,sourcekit-lsp,swiftpm,
 for %%I in (%SKIP_TESTS%) do (call set TestArg=%%TestArg:%%I,=%%)
 if "%TestArg:~-1%"=="," (set TestArg=%TestArg:~0,-1%) else (set TestArg= )