Consider replacing usage of built-in `TestCheckFunc` with state checks

[bendbennett]

Many of the tests within terraform-provider-corner make use of TestCheckFunc built-in functions from terraform-plugin-testing for verifying values in state. For instance, TestCheckResourceAttr:

func TestAccResourceUser(t *testing.T) {
	resource.UnitTest(t, resource.TestCase{
		ProtoV5ProviderFactories: map[string]func() (tfprotov5.ProviderServer, error){
			"tf5muxprovider": func() (tfprotov5.ProviderServer, error) {
				provider, err := New()

				return provider(), err
			},
		},
		Steps: []resource.TestStep{
			{
				Config: configResourceUserBasic,
				Check: resource.ComposeTestCheckFunc(
					resource.TestCheckResourceAttr("tf5muxprovider_user1.example", "age", "123"),
					/* ... */
				),
			},
		},
	})
}

Such tests could be updated to use the new style state checks that are now available, for example:

func TestAccResourceUser(t *testing.T) {
	resource.UnitTest(t, resource.TestCase{
		ProtoV5ProviderFactories: map[string]func() (tfprotov5.ProviderServer, error){
			"tf5muxprovider": func() (tfprotov5.ProviderServer, error) {
				provider, err := New()

				return provider(), err
			},
		},
		Steps: []resource.TestStep{
			{
				Config: configResourceUserBasic,
				ConfigStateChecks: []statecheck.StateCheck{
					statecheck.ExpectKnownValue(
						"tf5muxprovider_user1.example",
						tfjsonpath.New("age"),
						knownvalue.Int64Exact(123),
					),
					/* ... */
				},
			},
		},
	})
}