Code Modernization: Remove dynamic properties in `Tests_POMO_PO`.

In this particular case, the test class contains a `set_up()` method which sets properties only used by select tests, while the fixture method is being run for all tests.

There were a few ways this could be fixed:
* As the values do not change across tests, use `set_up_before_class()` to set (static) properties instead.
* For those values which are set up without a function call or variable interpolation, set the values in class constants.
* Or set these values as local variables for those tests which actually use them.

The implemented solution is a combination of options 2 and 3.

Follow-up to [1106/tests], [53557], [53558].

Props jrf.
See #56033.
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git-svn-id: http://core.svn.wordpress.org/trunk@53409 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53849';
+$wp_version = '6.1-alpha-53850';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.