Minefields spawn fix

[Night-Pryanik]

Summary

SUMMARY: Bugfixes "Made "minefield" map extra actually spawn."

Purpose of change

Closes #38289 - Minefields don't spawn.

Describe the solution

Previously minefield map extra's conditions were so strict that effectively it won't spawn at all. Every road overmap terrain had to roll one-in-75 = 1.33% check to decide whether this particular location should spawn map extra at all. If this 1-percent roll succeeded, an attempt to select mx_minefield map extra from a set is made. mx_minefield has a weight of 80, and there is plenty of other map extras with this weight or even larger in this set. If we're lucky to successfully roll these two checks on a particular road overmap terrain, we then need to check if this OT has a bridge nearby. The resulting chance to successfully all satisfy these three checks was very tiny. So I reworked the conditions to make this map extra actually spawn.

Created a separate bridge set of map extras and filled it with mx_minefield map extra with 20% chance to spawn on every bridge entry. Updated most coordinates in map extra's mapgen to reflect change from road to bridge spawning location.

Describe alternatives you've considered

None.

Testing

Teleported around map to find bridges, checked these bridges' entries for spawning of minefields.

Additional context

The chance to spawn (currently 20% per every bridge entry) could be lowered or increased if needed.

Thanks @ralreegorganon for looking into matter of this issue and explaining me how to fix it.

[jbytheway]

Hopefully the test in map_extra_test now passes reliably? If so, please remove the [!mayfail] tag therefrom.

[Night-Pryanik]

Yes, test passes now. Updated tests file, removed theoretical spawn case.

[ralreegorganon]

I don't see a reason to nuke the mx_minefield theorertical spawn case--just update the setup in it so that it sets up a valid case again, reflecting the changes you made for it to pass.