Ensure there is a default db prefix during PHPUnit tests (4.4)

[mollux]

Q	A
Bug fix? (use the a.b branch)	[x]
New feature/enhancement? (use the a.x branch)	[ ]
Deprecations?	[ ]
BC breaks? (use the c.x branch)	[ ]
Automated tests included?	[ ]
Related user documentation PR URL	mautic/mautic-documentation#...
Related developer documentation PR URL	mautic/developer-documentation#...
Issue(s) addressed	Fixes #...

Description:

In #11476 the db prefix was changed to be a fixed string by default.
But in the .env.dist file the MAUTIC_DB_PREFIX env var defaults to an empty string.
So all tests run without a prefix.

This PR ensures the default is also test_

Steps to test this PR:

1. Open this PR on Gitpod or pull down for testing locally (see docs on testing PRs here)

[codecov]

Codecov Report

Merging #11496 (69ba2ec) into 4.4 (6e81e89) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                4.4   #11496      +/-   ##
============================================
- Coverage     49.36%   49.36%   -0.01%     
  Complexity    35401    35401              
============================================
  Files          2144     2144              
  Lines        105534   105534              
============================================
- Hits          52102    52101       -1     
- Misses        53432    53433       +1     

Impacted Files	Coverage Δ
...eBundle/EventListener/DoctrineEventsSubscriber.php	72.72% <0.00%> (-1.82%)	⬇️

[escopecz]

Thanks!