tests: Add a test for SSL socket memory leaks.

Test is for an issue reported on the micropython-lib Discord as
effecting the rp2 port umqtt.simple interface when reconnecting with TLS,
however it's a more generic problem.

Currently this test fails on RPI_PICO_W and ESP32_GENERIC_C3 (and no doubt
others). Fixes are in the subsequent commits.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

tests/extmod/ssl_noleak.py

@@ -0,0 +1,50 @@
+# Ensure that SSLSockets can be allocated sequentially
+# without running out of available memory.
+try:
+    import io
+    import tls
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+import unittest
+
+
+class TestSocket(io.IOBase):
+    def write(self, buf):
+        return len(buf)
+
+    def readinto(self, buf):
+        return 0
+
+    def ioctl(self, cmd, arg):
+        return 0
+
+    def setblocking(self, value):
+        pass
+
+
+ITERS = 128
+
+
+class TLSNoLeaks(unittest.TestCase):
+    def test_unique_context(self):
+        for n in range(ITERS):
+            print(n)
+            s = TestSocket()
+            ctx = tls.SSLContext(tls.PROTOCOL_TLS_CLIENT)
+            ctx.verify_mode = tls.CERT_NONE
+            s = ctx.wrap_socket(s, do_handshake_on_connect=False)
+
+    def test_shared_context(self):
+        # Single SSLContext, multiple sockets
+        ctx = tls.SSLContext(tls.PROTOCOL_TLS_CLIENT)
+        ctx.verify_mode = tls.CERT_NONE
+        for n in range(ITERS):
+            print(n)
+            s = TestSocket()
+            s = ctx.wrap_socket(s, do_handshake_on_connect=False)
+
+
+if __name__ == "__main__":
+    unittest.main()