test: Increase Serial rx timeout

In the pre-release testing environment everything seems slower, and it
keeps failing with hitting this timeout.

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

tests/framework/microvm.py

@@ -865,7 +865,7 @@ def kill(self):
 class Serial:
     """Class for serial console communication with a Microvm."""
 
-    RX_TIMEOUT_S = 20
+    RX_TIMEOUT_S = 60
 
     def __init__(self, vm):
         """Initialize a new Serial object."""