riotctrl_shell.tests: add capability to access history of commands

dist/pythonlibs/riotctrl_shell/tests/common.py

@@ -10,17 +10,25 @@ import contextlib
 class MockSpawn():
     def __init__(self, ctrl, *args, **kwargs):
         self.ctrl = ctrl
-        self.last_command = None
+        self.commands = []
         # set some expected attributes
         self.before = None
         self.echo = False
 
+    @property
+    def last_command(self):
+        if self.commands:
+            return self.commands[-1]
+        else:
+            return None
+
     def read_nonblocking(self, size=1, timeout=-1):
         # do nothing, only used to flush pexpect output
         pass
 
     def sendline(self, line, *args, **kwargs):
-        self.last_command = line
+        if line:
+            self.commands.append(line)
         if self.ctrl.output is None:
             # just echo last input for before (what replwrap is assembling
             # output from)