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>