[analyzer] Fix check for response files

[ferkulat]

Instead of checking for @ in the file name, check if a file name
starts with @.
Fixes #3463.

Codechecker supports response files. These are files which you can pass to
clang to work around limits in command line length.
Those files names start with a @.
The CI/CD solution "Jenkins" creates workspaces by putting @ in the path name.
A file entry in compile_commands.json could look like:
"file": "/home/workspace/branchname@2/somefile.cpp"

In this case all files in the compile data base are treated as response file.
And the analyzer skips all files and does nothing.

With the help of @csordasmarton this got fixed by checking if a file starts with an @.

[csordasmarton]

Please add a test case for this change. Otherwise LGTM!

[csordasmarton]

Can you please add the following test case to https://github.com/Ericsson/codechecker/blob/master/analyzer/tests/unit/test_log_parser.py file:

    def test_source_file_contains_at_sign(self):
        """
        Test source file which path contains '@' sign in path.

        Source file path can contain '@' sign which doesn't mean it is a
        response file.
        """
        with tempfile.TemporaryDirectory(suffix='@') as tmp_dir:
            src_file_path = shutil.copy(self.src_file_path, tmp_dir)

            with open(self.compile_command_file_path, "w",
                      encoding="utf-8", errors="ignore") as f:
                f.write(json.dumps([dict(
                    directory=tmp_dir,
                    command=f"g++ {src_file_path}",
                    file=src_file_path
                )]))
            
        build_actions, _ = log_parser.parse_unique_log(load_json_or_empty(
            self.compile_command_file_path), self.__this_dir)

        self.assertEqual(len(build_actions), 1)

        build_action = build_actions[0]
        self.assertEqual(build_action.source, src_file_path)

Just to make sure if it works properly.

[csordasmarton]

LGTM!

[csordasmarton]

@ferkulat Thank you for this patch 😊