Increase timeout for flake

[gt-oai]

Saw:

stdout ───

    running 1 test
    test suite::shell_command::shell_command_works ... FAILED

    failures:

    failures:
        suite::shell_command::shell_command_works

    test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 513 filtered out; finished in 7.73s
    
  stderr ───

    thread 'suite::shell_command::shell_command_works' (13004) panicked at core\tests\common\lib.rs:27:28:
    regex "(?s)^Exit code: 0\\nWall time: [0-9]+(?:\\.[0-9]+)? seconds\\nOutput:\\nhello, world\\n?$" did not match "Exit code: 124\nWall time: 2 seconds\nOutput:\ncommand timed out after 2005 milliseconds\nhello, world"

So using MEDIUM_TIMEOUT:

/// Use this timeout if, empirically, a test seems to need more time than the
/// default.
const MEDIUM_TIMEOUT: Duration = Duration::from_secs(5);