Fixes random matrix:// unittest failures

[caronc]

Description:

Related issue (if applicable): #1162

matrix:// unit tests fail randomly. Heavy kudos to @fabiottini for finding that it's random calls to logout().

Since the NotifyMatrix() Plugin attempts to automatically close any established connections when the object is removed from memory, the randomness kind of hints that it's the garbage collection that is kicking in and freeing up an unhandled object later on.

This PR ensures that the matrix:// object is safely removed from memory after each unit test which SHOULD prevent the randomness from occuring in the future.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 100% test coverage

Testing

Anyone can help test this source code as follows:

# Create a virtual environment to work in as follows:
python3 -m venv apprise

# Change into our new directory
cd apprise

# Activate our virtual environment
source bin/activate

# Install the branch
pip install git+https://github.com/caronc/apprise.git@random-matrix-test-failures

# Test out the changes with the following command:
bin/test matrix