ExternalConsoleDriver: fix flaky txchunk test

The txchunk test case added in [1] seems to fail occasionally.

Fix this this flaky behavior by adding delays between writing and
reading bytes, similar to what was being done in the `test_communicate`
function added in commit [2].

[1] 5eb9316 ("driver: introduce txchunk argument for console drivers")
[2] 69d5c69 ("ExternalConsoleDriver: Add a ConsoleDriver for existing console tools")

Signed-off-by: Fabian Pfitzner <f.pfitzner@pengutronix.de>

Author: N3xp7im3

tests/test_externalconsoledriver.py

@@ -27,14 +27,17 @@ def test_chunking(self, target):
         d.txchunk = 2
         d.txdelay = 0.01
         d.write(data)
+        time.sleep(0.1)
         assert d.read(len(data)) == data  # assert written data is read
 
         data = b"odd sized\ndata."
         d.write(data)
+        time.sleep(0.1)
         assert d.read(len(data)) == data  # assert written data is read
 
         data = b"odd chunk\nsize."
         d.txchunk = 3
         d.write(data)
+        time.sleep(0.1)
         assert d.read(len(data)) == data  # assert written data is read
         d.close()