Add tests against local Blazegraph

[vemonet]

This PR adds tests to check if a local instance of blazegraph is running, and if so, see if federation works against it. If a local instance of Blazegraph isn't running, these tests are skipped.

There was also the idea that the Python code should take care of downloading the Blazegraph jar and running it in the background, but this proved to be pretty tough, so it's been tabled for this PR. here are some notes on what was tried:

def _start_blazegraph(path):
    import subprocess

    proc_blazegraph = Process(
        target=subprocess.run,
        args=([f"java -jar {path}"]),
        kwargs={
            "shell": True,
        },
        daemon=True,
    )
    proc_blazegraph.start()
    time.sleep(1)
    return proc_blazegraph

    # TODO: blazegraph not actually killed, it will continue to run afterward
    # Not sure how to make sure a java subprocess running as daemon is killed
    # self.proc_blazegraph.kill()

    # manual instructions for cancelling process:
    # 1. shell: ps aux | grep blaze
    # 2. find the number of the process
    # 3. shell: kill #

[codecov-commenter]

Codecov Report

Merging #49 (ff1afe9) into main (da58e65) will not change coverage.
The diff coverage is n/a.

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##              main       #49   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            8         8           
  Lines          479       479           
  Branches       103       103           
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  Hits           479       479           

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[cthoyt]

@vemonet I've now simplified this down to run only if there's already a local blazegraph available - this takes the burden of writing code to run blazegraph out of the code itself (for now). I saved some of the notes on how to potentially update this in this PR's description