exercise util: fix update cmd test file layout

[coriolinus]

The test file is easiest for a human to read and understand if it contains the following sections in sequence:

• test module doccomment
• imports
• process_X_case implementations
• tests

The exercise update command should ensure that the test output file conforms to that sequencing.

From the exercise tool's point of view, https://github.com/exercism/rust/issues/685 requires parsing the test file to determine which tests exist, and what bytes they contain, anyway. Given that information, it should not be difficult to insert new process_X_case implementations at the end of the appropriate section.