Added and corrected test coverage

Author: JoshuaKGoldberg

index.test-d.ts

@@ -57,7 +57,7 @@ expectType<{
   defaultOne: string,
   defaultTwo: number,
   version: string,
-}>(BaseLevelTwo.defaultOptions);
+}>({ ...BaseLevelTwo.defaultOptions });
 
 // Because 'version' is already provided, this needs no argument
 new BaseLevelTwo();
@@ -88,7 +88,7 @@ expectType<{
   defaultTwo: number,
   defaultThree: string[],
   version: string,
-}>(BaseLevelThree.defaultOptions);
+}>({ ...BaseLevelThree.defaultOptions });
 
 // Because 'version' is already provided, this needs no argument
 new BaseLevelThree();

test/base.test.js

@@ -33,10 +33,20 @@ test(".defaults({foo: 'bar'})", () => {
   const BaseWithDefaults = Base.defaults({ foo: "bar" });
   const defaultsTest = new BaseWithDefaults();
   const mergedOptionsTest = new BaseWithDefaults({ baz: "daz" });
+  assert.equal(BaseWithDefaults.defaultOptions, { foo: "bar" });
   assert.equal(defaultsTest.options, { foo: "bar" });
   assert.equal(mergedOptionsTest.options, { foo: "bar", baz: "daz" });
 });
 
+test(".defaults({foo: 'bar', baz: 'daz' })", () => {
+  const BaseWithDefaults = Base.defaults({ foo: "bar" }).defaults({ baz: "daz" });
+  const defaultsTest = new BaseWithDefaults();
+  const mergedOptionsTest = new BaseWithDefaults({ faz: "boo" });
+  assert.equal(BaseWithDefaults.defaultOptions, { foo: "bar", baz: "daz" });
+  assert.equal(defaultsTest.options, { foo: "bar", baz: "daz" });
+  assert.equal(mergedOptionsTest.options, { foo: "bar", baz: "daz", faz: "boo" });
+});
+
 test(".plugin().defaults()", () => {
   const BaseWithPluginAndDefaults = Base.plugin(fooPlugin).defaults({
     baz: "daz",