test/threads: fix state leakage across tests

test_threaded_bad_options was only passing in CI because it was running
after test_threaded_invalid_binary, which altered the base class to
ignore some exceptions.

Restructure the tests by using the fact that ThreadedAustin.join will
bubble up any exception that happened during startup. This avoids
manipulating global state in test_threaded_invalid_binary.

Fixes P403n1x87#13.

test/test_threads.py

@@ -72,10 +72,6 @@ def assert_callbacks_called(self):
 class InvalidBinaryThreadedAustin(ThreadedAustin):
     BINARY = "_austin"
 
-    @check_raises(AustinError)
-    def run(self, *args, **kwargs):
-        super().run(*args, **kwargs)
-
 
 def test_threaded():
     austin = TestThreadedAustin()
@@ -109,8 +105,10 @@ def terminate_callback(*args):
 
 
 def test_threaded_invalid_binary():
-    SimpleAustin.start = check_raises(AustinError)(SimpleAustin.start)
-    InvalidBinaryThreadedAustin(sample_callback=lambda x: None).start(["python"])
+    austin = InvalidBinaryThreadedAustin(sample_callback=lambda x: None)
+    austin.start(["python"])
+    with raises(AustinError):
+        austin.join()
 
 
 def test_threaded_no_sample_callback():
@@ -121,4 +119,5 @@ def test_threaded_no_sample_callback():
 def test_threaded_bad_options():
     austin = TestThreadedAustin(terminate_callback=lambda *args: None)
     austin.start(["-I", "1000", "python", "-c", "for i in range(1000000): print(i)"])
-    austin.join()
+    with raises(AustinError):
+        austin.join()