tests: Run nxdtest.main for each test in a separate thread, so that pytest.timeout generally works

Factor-out the driver to run_nxdtest in a separate thread done in
0ad45a9c (Detect if a test leaks processes and terminate them) from
test_run_procleak into run_nxdtest, so that all tests are run this way
for pytest.timeout, if requested, works universally for all tests.

The logic of how we run nxdtest under tests and handle timeout will be,
hopefully, soon reworked more, but it is anyway good to keep this logic
in only one place.

nxdtest/nxdtest_test.py

@@ -38,16 +38,38 @@ def run_nxdtest(tmpdir):
     passed as `argv`.
     """
 
+    @func
     def _run_nxdtest(nxdtest_file_content, argv=("nxdtest",)):
         with tmpdir.as_cwd():
             with open(".nxdtest", "w") as f:
                 f.write(nxdtest_file_content)
             sys_argv = sys.argv
             sys.argv = argv
-            try:
-                main()
-            finally:
+            def _():
                 sys.argv = sys_argv
+            defer(_)
+
+            # run nxdtest in thread so that timeout handling works
+            # ( if nxdtest is run on main thread, then non-py wait in WorkGroup.wait, if
+            #   stuck, prevents signals from being handled at python-level )
+            wg = sync.WorkGroup(context.background())
+            done = chan()
+            @func
+            def _(ctx):
+                defer(done.close)
+                main()
+            wg.go(_)
+
+            while 1:
+                _, _rx = select(
+                            default,    # 0
+                            done.recv,  # 1
+                )
+                if _ == 0:
+                    time.sleep(0.1)
+                    continue
+                wg.wait()
+                break
 
     return _run_nxdtest
 
@@ -134,32 +156,11 @@ TestCase('TEST10', ['echo', 'TEST10'])
 def test_run_procleak(run_nxdtest, capsys):
     procleak = "%s/testprog/procleak" % (dirname(__file__),)
 
-    # run nxdtest in thread so that timeout handling works
-    # ( if nxdtest is run on main thread, then non-py wait in WorkGroup.wait, if
-    #   stuck, prevents signals from being handled at python-level )
-    wg = sync.WorkGroup(context.background())
-    done = chan()
-    @func
-    def _(ctx):
-        defer(done.close)
-
     run_nxdtest(
         """\
 TestCase('TEST_WITH_PROCLEAK', ['%s', 'AAA', 'BBB', 'CCC'])
 """     % procleak
     )
-    wg.go(_)
-
-    while 1:
-        _, _rx = select(
-                    default,    # 0
-                    done.recv,  # 1
-        )
-        if _ == 0:
-            time.sleep(0.1)
-            continue
-        wg.wait()
-        break
 
     captured = capsys.readouterr()
     assert "AAA: terminating" in captured.out