Depot.java:20-22: Add unit tests for `Depot`. We should...

[0pdd]

The puzzle 9-b46ddcfe from #9 has to be resolved:

sodg-maven-plugin/src/main/java/org/eolang/sodg/Depot.java

Lines 20 to 22 in 366715e

	* @todo #9:35min Add unit tests for `Depot`.
	* We should test the trains inside of it, and method `train()`, that supposed
	* to return proper train by it's name. Don't forget to remove this puzzle.

The puzzle was created by @yegor256 on 03-Oct-25.

Estimate: 35 minutes, role: DEV.

If you have any technical questions, don't ask me, submit new tickets instead. The task will be "done" when the problem is fixed and the text of the puzzle is removed from the source code. Here is more about PDD and about me.