test: add tests for calculating the spell price

spec/setup.ts

@@ -4,6 +4,35 @@
 
 import en from "../lang/en.json";
 
+function setupPrimitives() {
+    Object.defineProperties(Number, {
+        isNumeric: {
+            value: function (n: unknown) {
+                if (n instanceof Array) return false;
+                else if (([null, ""] as unknown[]).includes(n)) return false;
+                // @ts-expect-error Abusing JavaScript a bit here, but it's the implementation from foundry
+                return +n === +n;
+            },
+        },
+        fromString: {
+            value: function (str: unknown) {
+                if (typeof str !== "string" || !str.length) return NaN;
+                // Remove whitespace.
+                str = str.replace(/\s+/g, "");
+                return Number(str);
+            },
+        },
+    });
+
+    Object.defineProperties(Math, {
+        clamped: {
+            value: function (num: number, min: number, max: number) {
+                return Math.min(max, Math.max(num, min));
+            },
+        },
+    });
+}
+
 function setupStubs() {
     class StubGame {
         constructor() {
@@ -24,4 +53,5 @@ function setupStubs() {
     });
 }
 
+setupPrimitives();
 setupStubs();