Discounts can only be used one time (#892)

## Description:

Discounts on structures and warships can only be used one time.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

---------

Co-authored-by: Scott Anderson <scottanderson@users.noreply.github.com>

src/core/execution/FakeHumanExecution.ts

@@ -466,8 +466,7 @@ export class FakeHumanExecution implements Execution {
 
   private maybeSpawnStructure(type: UnitType, maxNum: number): boolean {
     if (this.player === null) throw new Error("not initialized");
-    const units = this.player.unitsIncludingConstruction(type);
-    if (units.length >= maxNum) {
+    if (this.player.unitsOwned(type) >= maxNum) {
       return false;
     }
     if (this.player.gold() < this.cost(type)) {