plot creation tests should not rely on access to /apophis

[carrholt]

The test setup is relying on access to a directory that it shouldn't need

`======================================================================
ERROR: test_plot_gallonglat (core.tests.test_plots.TestMakeVisibilityPlot)

Traceback (most recent call last):
File "/Users/carrieholt/git/neoexchange/neoexchange/core/tests/test_plots.py", line 157, in test_plot_gallonglat
response = make_visibility_plot(None, self.test_body.pk, 'glonglat', self.start_time)
File "/Users/carrieholt/git/neoexchange/neoexchange/core/plots.py", line 173, in make_visibility_plot
os.makedirs(os.path.join(default_storage.base_location, base_dir))
File "/Users/carrieholt/micromamba/envs/neoexchange/lib/python3.9/os.py", line 215, in makedirs
makedirs(head, exist_ok=exist_ok)
File "/Users/carrieholt/micromamba/envs/neoexchange/lib/python3.9/os.py", line 215, in makedirs
makedirs(head, exist_ok=exist_ok)
File "/Users/carrieholt/micromamba/envs/neoexchange/lib/python3.9/os.py", line 215, in makedirs
makedirs(head, exist_ok=exist_ok)
[Previous line repeated 1 more time]
File "/Users/carrieholt/micromamba/envs/neoexchange/lib/python3.9/os.py", line 225, in makedirs
mkdir(name, mode)
OSError: [Errno 30] Read-only file system: '/apophis'`