fix(get_app_version): work as intended, add tests

[malept]

What this PR does / why we need it

Porting over the Magefile version of get_app_version did not work as intended, and it's unclear why.

• Removed --always from git describe so that a repo without any tags actually uses the fallback.
• Updated the value passed to --match to be a glob instead of a regex, per the docs.
• Minor change: prefix fallback version with v for consistency

Also adds tests to show what it's supposed to do.

Jira ID

DT-4561

Notes for your reviewers

--always documentation from git describe --help:

       --always
           Show uniquely abbreviated commit object as fallback.

[richardlarocque]

LGTM.

[getoutreach-ci-2]

Link to code coverage report (posted by coverbot 🤖)

[ekalinichev]

There are unit tests for Bash? Nice )

[getoutreach-ci-1]

🎉 This PR is included in version 2.29.0-rc.8 🎉

The release is available on:

• GitHub release
• CircleCI Orb

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