twister: fix shell prompt detection with VT100 colors disabled

Device adapter strips all whitespace from output lines causing test
failures when matching default shell prompt "uart:~$ " with trailing
space. Update _handle_device_output to only strip line endings (\r\n)
while preserving whitespace required for prompt detection.

A testcase sample.pytest.shell.no_vt100 was added to verify prompt
matching works with CONFIG_SHELL_VT100_COLORS disabled.

Signed-off-by: Thomas Günther <thomas.guenther@limatica.com>

samples/subsys/testsuite/pytest/shell/testcase.yaml

@@ -12,3 +12,17 @@ tests:
       - test_framework
       - pytest
       - shell
+  sample.pytest.shell.vt100_colors_off:
+    filter: CONFIG_SERIAL and dt_chosen_enabled("zephyr,shell-uart")
+    min_ram: 40
+    harness: pytest
+    extra_configs:
+      - arch:posix:CONFIG_NATIVE_UART_0_ON_STDINOUT=y
+      - CONFIG_SHELL_VT100_COLORS=n
+    integration_platforms:
+      - native_sim
+      - qemu_cortex_m3
+    tags:
+      - test_framework
+      - pytest
+      - shell

scripts/pylib/pytest-twister-harness/src/twister_harness/device/device_adapter.py

@@ -240,7 +240,7 @@ class DeviceAdapter(abc.ABC):
         with open(self.handler_log_path, 'a+') as log_file:
             while self.is_device_running():
                 if self.is_device_connected():
-                    output = self._read_device_output().decode(errors='replace').strip()
+                    output = self._read_device_output().decode(errors='replace').rstrip("\r\n")
                     if output:
                         self._device_read_queue.put(output)
                         log_file.write(f'{output}\n')