[transactional-test-runner] Document how run_test compares the output of a .move file against an expected output .exp

[miker83z]

Stems from #1979 (comment)

Document that the output produced by each .move file will get matched against the expected output defined in the respective .exp file:

pub async fn run_test_impl<'a, Adapter>(
    path: &Path,
    fully_compiled_program_opt: Option<Arc<FullyCompiledProgram>>,
) -> Result<(), Box<dyn std::error::Error>>
where
    Adapter: MoveTestAdapter<'a>,
    Adapter::ExtraInitArgs: Debug,
    Adapter::ExtraPublishArgs: Debug,
    Adapter::ExtraValueArgs: Debug,
    Adapter::ExtraRunArgs: Debug,
    Adapter::Subcommand: Debug,
{
    let output = handle_actual_output::<Adapter>(path, fully_compiled_program_opt).await?;
    handle_expected_output(path, output.0)?;
    Ok(())
}

• This is independent of subcommands and can fit into the general section
• Describe the whole process
• Describe the structure of the .move file
• Describe the structure of the .exp file