Re-added test file + ran biome

test/@types/test-helpers.ts

@@ -0,0 +1,27 @@
+import type { AtLeastOne, NonFunctionPropertiesRecursive as nonFunc } from "#types/type-helpers";
+
+/**
+ * Helper type to admit an object containing the given properties
+ * _and_ at least 1 other non-function property.
+ * @example
+ * ```ts
+ * type foo = {
+ *   qux: 1 | 2 | 3,
+ *   bar: number,
+ *   baz: string
+ *   quux: () => void; // ignored!
+ * }
+ *
+ * type quxAndSomethingElse = OneOther<foo, "qux">
+ *
+ * const good1: quxAndSomethingElse = {qux: 1, bar: 3} // OK!
+ * const good2: quxAndSomethingElse = {qux: 2, baz: "4", bar: 12} // OK!
+ * const bad1: quxAndSomethingElse = {baz: "4", bar: 12} // Errors because `qux` is required
+ * const bad2: quxAndSomethingElse = {qux: 1} // Errors because at least 1 thing _other_ than `qux` is required
+ * ```
+ * @typeParam O - The object to source keys from
+ * @typeParam K - One or more of O's keys to render mandatory
+ */
+export type OneOther<O extends object, K extends keyof O> = AtLeastOne<Omit<nonFunc<O>, K>> & {
+  [key in K]: O[K];
+};