chore(dtslint): add find tests

Closes ReactiveX#3969.

spec-dtslint/operators/find-spec.ts

@@ -0,0 +1,26 @@
+import { of } from 'rxjs';
+import { find } from 'rxjs/operators';
+
+it('should support a user-defined type guard', () => {
+  const o = of('foo').pipe(find((s: string): s is 'foo' => true)); // $ExpectType Observable<"foo" | undefined>
+});
+
+it('should support a user-defined type guard that takes an index', () => {
+  const o = of('foo').pipe(find((s, index): s is 'foo' => true)); // $ExpectType Observable<"foo" | undefined>
+});
+
+it('should support a user-defined type guard that takes an index and the source', () => {
+  const o = of('foo').pipe(find((s, index): s is 'foo' => true)); // $ExpectType Observable<"foo" | undefined>
+});
+
+it('should support a predicate', () => {
+  const o = of('foo').pipe(find(s => true)); // $ExpectType Observable<string | undefined>
+});
+
+it('should support a predicate that takes an index', () => {
+  const o = of('foo').pipe(find((s, index) => true)); // $ExpectType Observable<string | undefined>
+});
+
+it('should support a predicate that takes an index and the source', () => {
+  const o = of('foo').pipe(find((s, index, source) => true)); // $ExpectType Observable<string | undefined>
+});