Reorganize tests into separate modules

[nfahlgren]

Describe your changes
Reorganizes tests into individual modules instead of one large tests file. Streamlines/reorganizes test data to remove unnecessary/redundant test data in favor of smaller/faster test samples. Updates tests to use parameters and test a single function per test.

Type of update
Is this a: New feature or feature enhancement

[nfahlgren]

@HaleySchuhl @JorgeGtz this PR now re-implements our tests with 100% coverage but split into smaller modules, feel free to take a look when you have time. There are a lot of files changed but most of those changes are:

• Addition of new test module files
• Moving data from the original tests data directories to the unified testdata directory
• Removal of test data that is no longer used/needed

There are a few minor updates to some PlantCV functions that should just refactor the code to remove the need for a few tests (e.g. removed all cases of if params.debug is not None).

So other than these few latter cases, most of the other file changes don't need to be reviewed thoroughly since they are just cleaned up tests moved from tests.py into modules.

One last thing, the empty file tests.py is still included because I think that will help with conflict resolution as we merge these changes into existing PRs. We can remove it later after we've dealt with those conflicts.