plugin/battery: fix handling of multiple batteries with upower

[gaelicWizard]

Description

This should fix both our BATS tests as well as the battery_percentage() (and related) function(s) to properly handle multiple attached batteries, and closes #2104.

Motivation and Context

The previous fix for multiple batteries returned by upower -e just tried to snip off the first line and run with it, but this seems to have triggered a bug in Bash related to set -o pipefail (which is enabled by BATS during testing, and elsewhere).

This patch changes plugin/battery to just read all the batteries into an array, thus not cutting off the enumeration prematurely and avoiding the testing bug.

Currently, this patch does not handle any batteries after the first, but at least it doesn't just trim the list.

How Has This Been Tested?

locally, and GitHub Actions

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[NoahGorny]

Nice fix!