Generate unit test line by line (for --inplace)

[jaisnan]

Description of changes:

This PR adds -

1. Generating the unit tests (concrete playback tests) line by line using a sync buffered reader/writer.
2. Util functions and related unit tests.
3. Minor refactoring

The reason to generate the unit tests line by line is -

1. to avoid storing the entire unit test, no matter how large in a string variable before dumping it.
2. To make sure users still have access to the generated unit tests in case generation fails midway.

Resolved issues:

Closes the changes proposed by #1625

Call-outs:

1. Uses the tempfile crate to take care of the file handling. The documentation says that the tempfile is deleted automatically whenever it goes out of scope.
2. Changes the definition of the UnitTest struct.
3. There are no regression tests for the changes made.

Testing:

• How is this change tested? unit tests but not

• Is this a refactor change? yes

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[celinval]

Thanks

[celinval]

It's looking good. I was wondering if we should modify the TempFile slightly. So, we don't want to delete the file if it doesn't exist anymore. We should also consider bringing the BufWriter to be inside this structure since we want to explicitly call flush() before dropping it and before removing the file.

[celinval]

Awesome, thanks again. Not for this PR, but we should eventually change this code to write all the unit tests in the file at once.

[celinval]

Thanks @jaisnan. Just tiny comments