scripts: pytest: Add option to filter the captured shell output

Sometimes you might need exec_command() to return an value from
running device. If the output contains shell prompts or logger
output, it is harder to capture the value.
Filter away the lines we thing belongs to the shell prompt or
normal logging output.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>

scripts/pylib/pytest-twister-harness/src/twister_harness/helpers/shell.py

@@ -66,3 +66,15 @@ class Shell:
         # wait for device command execution
         lines.extend(self._device.readlines_until(regex=regex_prompt, timeout=timeout, print_output=print_output))
         return lines
+
+    def get_filtered_output(self, command_lines: list[str]) -> list[str]:
+        regex_filter = re.compile(
+            '|'.join([
+                re.escape(self.prompt),
+                '<dbg>',
+                '<inf>',
+                '<wrn>',
+                '<err>'
+            ])
+        )
+        return list(filter(lambda l: not regex_filter.search(l), command_lines))