fix _check_prompt with interjected logs

Author: LukasLendvorsky

labgrid/driver/shelldriver.py

@@ -197,9 +197,13 @@ def _check_prompt(self):
         self.console.sendline(f"echo '{marker[:4]}''{marker[4:]}'")
         try:
             self.console.expect(
-                rf"{marker}\s+{self.prompt}",
+                rf"{marker}\s",
                 timeout=30
             )
+            self.console.expect(
+                rf"{self.prompt}",
+                timeout=1
+            )
             self._status = 1
         except TIMEOUT:
             self._status = 0