tests/pyb: Add test for ExtInt when doing swint while disabled.

tests/pyb/extint.py

@@ -1,8 +1,17 @@
 import pyb
 
+# test basic functionality
 ext = pyb.ExtInt('X1', pyb.ExtInt.IRQ_RISING, pyb.Pin.PULL_DOWN, lambda l:print('line:', l))
 ext.disable()
 ext.enable()
 print(ext.line())
 ext.swint()
+
+# test swint while disabled, then again after re-enabled
+ext.disable()
+ext.swint()
+ext.enable()
+ext.swint()
+
+# disable now that the test is finished
 ext.disable()