Add in copyright tests to rqt_plot.

[clalancette]

There are 2 reasons to do this:

1. All packages really should enforce copyright and license notices; this PR brings us into compliance with the rest of our code.
2. By having at least one test here, we work around the fact that we can't import PyQt on Windows Debug. In particular, if you don't specify anything, then pytest will attempt to import everything and find tests. By specifically telling it where to find tests with a pytest.ini file, we ensure it only ever runs there. To lift this restriction in the future we'd have to figure out how to make PyQt work on Windows Debug.

[clalancette]

The combination of this and ros-visualization/rqt_bag#154 should fix the build on Windows Debug. @claraberendsen FYI

[clalancette]

CI:

• Linux
• Linux-aarch64
• Windows
• Windows Debug