[Uptime] FilterGroup component is untested and has accessibility issues

[justinkambic]

Kibana version:
edge

Elasticsearch version:
edge

Server OS version:
macOS

Browser version:
Chrome 87

Browser OS version:
macOS

Original install method (e.g. download page, yum, from source, etc.):
from source

Describe the bug:
The FilterGroup component in the Uptime app is untested, and several key elements are missing aria-labels which, if added, could significantly improve the UX for users relying on these fields.

Steps to reproduce:

1. Attempt to run the test suite for FilterGroup
2. There isn't one

Expected behavior:
There should be unit tests and appropriate accessibility labeling.

Screenshots (if relevant):
N/A

Errors in browser console (if relevant):
N/A

Provide logs and/or server output (if relevant):
N/A

Any additional context:

[elasticmachine]

Pinging @elastic/uptime (Team:uptime)