docs(testing): add assertExists example (#9613)

docs/testing/assertions.md

@@ -16,6 +16,7 @@ The assertions module provides 10 assertions:
 
 - `assert(expr: unknown, msg = ""): asserts expr`
 - `assertEquals(actual: unknown, expected: unknown, msg?: string): void`
+- `assertExists(actual: unknown,msg?: string): void`
 - `assertNotEquals(actual: unknown, expected: unknown, msg?: string): void`
 - `assertStrictEquals(actual: unknown, expected: unknown, msg?: string): void`
 - `assertStringIncludes(actual: string, expected: string, msg?: string): void`
@@ -39,6 +40,19 @@ Deno.test("Test Assert", () => {
 });
 ```
 
+### Exists
+
+The `assertExists` can be used to check if a value is not `null` or `undefined`.
+
+```js
+assertExists("Denosaurus");
+Deno.test("Test Assert Exists", () => {
+  assertExists("Denosaurus");
+  assertExists(false);
+  assertExists(0);
+});
+```
+
 ### Equality
 
 There are three equality assertions available, `assertEquals()`,