Use mypy to check the tests dir; fix some typing errors

[bradlarsen]

Fixes #1608.

Additionally, this includes a few type annotations in a couple other places, which I added as I was looking there.

[ekilmer]

Very nice! 💯

[ehennenfent]

Speculating from the CI output, but it looks like MockMem and Memory don't behave quite the same way, which is causing the native tests to fail?

Also, I've consolidated the testing and tests labels into the former.

[bradlarsen]

Speculating from the CI output, but it looks like MockMem and Memory don't behave quite the same way, which is causing the native tests to fail?

Previously, the use of Memory already referred to mockmem.MockMemory.
The previous change (Memory -> MockMemory) shouldn't make a runtime difference; it merely avoids the shadowed import that mypy didn't like.

The test failure, in this case, was actually due to me incorrectly fixing this bad format string the first time around:

diff --git a/tests/native/test_memory.py b/tests/native/test_memory.py
index ac1f6d3..c74254b 100644
--- a/tests/native/test_memory.py
+++ b/tests/native/test_memory.py
@@ -1428,7 +1428,7 @@ class MemoryTest(unittest.TestCase):
         # No Access Reading <4160741376>
         # self.assertRaisesRegexp(MemoryException, r"No access reading.*", mem.__getitem__, x)
         with self.assertRaisesRegex(
-            InvalidSymbolicMemoryAccess, "Invalid symbolic memory access.*".format(addr)
+            InvalidSymbolicMemoryAccess, f"Invalid symbolic memory access.*<{addr:x}>"
         ):
             _ = mem[x]
             # mem[addr] = 'a'

Also, I've consolidated the testing and tests labels into the former.

Thanks!