[lldb] Remove tab from TestReturnValue.py

Mixing tabs and spaces makes Python exit with this error:

  File "llvm/lldb/packages/Python/lldbsuite/test/functionalities/return-value/TestReturnValue.py", line 23
    return (self.getArchitecture() == "aarch64" and self.getPlatform() == "linux")
                                                                                 ^
TabError: inconsistent use of tabs and spaces in indentation