Run single process tests on Windows CI to avoid some permission errors.

.ci/azp-windows-test.yml

@@ -26,7 +26,7 @@ steps:
     $env:path = 'C:\Windows\system32;C:\Windows;' + $env:CXX_PATH
     cd test
     echo "using" $env:TEST_TOOLSET ":" ":" $env:CXX ";" > ${env:HOME}/user-config.jam
-    py -X utf8 test_all.py $env:TEST_TOOLSET
+    py -X utf8 test_all.py $env:TEST_TOOLSET --not-parallel
     cd ..
   displayName: Test
 - powershell: |

test/test_all.py

@@ -73,7 +73,8 @@ def run_tests(critical_tests, other_tests):
             max_test_name_len = len(x)
 
     cancelled = False
-    executor = concurrent.futures.ProcessPoolExecutor()
+    max_workers = 1 if "--not-parallel" in sys.argv else None
+    executor = concurrent.futures.ProcessPoolExecutor(max_workers=max_workers)
 
     def handler(sig, frame):
         cancelled = True