fix: Enhance platform check for stackql installation(macOS)

[derek10cloud]

This PR enhances the reliability of stackql installation on MacOS + Fix Test Codes by:

1) Modifying the platform check to use startswith('Darwin') instead of strict equality. This ensures compatibility with various Darwin-based OS versions.

When _setup function check the platform, MacOS user's platform is not excatly Darwin,
but it just starts with Darwin, so I changed the condition to check if platform.startswith('Darwin').

2) Adding a step to clear any existing files in the unpacked directory before extracting the new stackql binary. This prevents potential conflicts and ensures a clean installation.

[before]

command = 'pkgutil --expand-full {} {}'.format(archive_file_name, unpacked_file_name)
os.system(command)

command is like pkgutil --expand-full /Users/derek/.local/stackql_darwin_multiarch.pkg /Users/derek/.local/stackql

• /Users/derek/.local/stackql_darwin_multiarch.pkg(archive_file_name): This is the path to your macOS package file.
• /Users/derek/.local/stackql(unpacked_file_name): This is the directory where the package's contents will be extracted.
  But if I didn't remove unpacked_file_name, there was an error like below,

So I add remove existing unpacked_file_name to make a new unpacked_file_name.
After I apply this logic, there are no error like above.

(Sorry for debugging message..!)

But if there are any other options instead of deleting unpacked_file_name, let me know any time 😃

3) Removing External Dependencies: Eliminated the reliance on the test.env file during test execution. This enables me to test locally without test.env file

[before]

[after]
After I deleted . tests/creds/env_vars/test.env execution logic, there is no error.
Instead, I changed os.environ[VARIABLES] to simple string value.

4) Introducing Mocking for Server Mode Tests: Added mocking logic to simulate the behavior of pystackql.StackQL methods in server mode.

[before]
There were errors, because it tried to execute real methods such as pystackql.StackQL.execute, pystackql.stackql.StackQL._run_server_query.

[After]
After I mock the method and the response, it doesn't make any error.