Improve test_load_multiple_days

[aburrell]

Is your feature request related to a problem? Please describe.
Not all data sets will have multiple days of data in a row.

Describe the solution you'd like
Evaluate the available downloaded files and ensure this test can be run

Describe alternatives you've considered
Current solution is to change dates/not run this until the next release

Additional context
In most cases a better test date can be found, but this may not always be true.

[jklenzing]

Might need to make some tests optional. Need to add "opt out" flags for instruments for certain tests. For example, rockets will only have one day of data.