Explain the use of Go's built-in testing package for unit testing?

The built-in testing package is a popular way to write and run unit tests in Go. The package provides a framework for writing tests, including functions for defining test cases, running tests, and reporting results.

Here is an example of a simple test case using the testing package:

func TestAdd(t *testing.T) {
    result := Add(2, 3)
    if result != 5 {
        t.Errorf("Expected 5 but got %d", result)
    }
}

In this example, **TestAdd** is a test function that takes a **testing.T** parameter. The function calls the **Add** function and checks whether the result is equal to the expected value of 5. If the result is not as expected, the test function calls **t.Errorf** to report a failure.

To run tests using the testing package, you can use the **go test** command. By default, this command will search for any test files in the current directory and subdirectories and run all tests that it finds.

In addition to the basic functionality shown in this example, the testing package provides many other features for writing tests in Go, including test fixtures, subtests, and benchmarks.