[windows] improve SKIP_TESTS substitution

Author: charles-zablit

utils/build-windows-toolchain.bat

@@ -62,9 +62,15 @@ 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,lldb-swift,swift,dispatch,foundation,xctest,swift-format,sourcekit-lsp,
-for %%I in (%SKIP_TESTS%) do (call set TestArg=%%TestArg:%%I,=%%)
-if "%TestArg:~-1%"=="," (set TestArg=%TestArg:~0,-1%) else (set TestArg= )
+set TestsList=lld,lldb,lldb-swift,swift,dispatch,foundation,xctest,swift-format,sourcekit-lsp
+set "TestArg="
+set "Skip=,%SKIP_TESTS%,"
+for %%I in (%TestsList%) do (
+  if "!Skip:,%%I,=!" == "!Skip!" (
+      set "TestArg=!TestArg!%%I,"
+  )
+)
+set "TestArg=-Test !TestArg!"
 
 :: Build the packaging arguments (skipped for normal PRs and an added stage for toolchain PRs)
 set "PackagingArg=-SkipPackaging"