Allow running subsequent integration test builds

[Malax]

To achieve a consistent API, breaking changes had to be made to allow sharing of TestConfig (formerly IntegrationTest) for both initial and subsequent test runs. The biggest change is the addition of TestRunner which is now the way to run tests.

Even though it's a breaking change, migration to this new API should be simple. See examples in this PR for an idea of required changes.

Changelog

• Remove IntegrationTest::run_test, to run a test use the new TestRunner::run_test function.
• Rename IntegrationTest to TestConfig.
• Rename IntegrationTestContext to TestContext.
• Add Clone implementation for TestConfig, allowing it to be shared across tests.
• Add TestContext::run_test and TestContext::run_test_inherit_config, allowing you to run subsequent integration tests with the image from a previous test. These functions allow testing of subsequent builds, including caching logic and buildpack behaviour when build environment variables change, stacks are upgraded and more.

Examples

#[test]
fn basic() {
    TestRunner::default().run_test(
        TestConfig::new("heroku/buildpacks:20", "test-fixtures/simple-ruby-app"),
        |context| {
            assert_contains!(context.pack_stdout, "---> Ruby Buildpack");
            assert_contains!(context.pack_stdout, "---> Installing bundler");
            assert_contains!(context.pack_stdout, "---> Installing gems");

            context.run_test_inherit_config(|context| {
                assert_not_contains!(context.pack_stdout, "---> Installing bundler");
                assert_not_contains!(context.pack_stdout, "---> Installing gems");
            });
        },
    );
}

#[test]
fn clear_bundler_cache() {
    let base_config = TestConfig::new("heroku/buildpacks:20", "test-fixtures/simple-ruby-app");

    TestRunner::default().run_test(&base_config, |context| {
        assert_contains!(context.pack_stdout, "---> Ruby Buildpack");
        assert_contains!(context.pack_stdout, "---> Installing bundler");
        assert_contains!(context.pack_stdout, "---> Installing gems");

        context.run_test(
            // Subsequent tests can be run using a different config, allowing testing of stack
            // changes, different environment variables, etc. Since TestConfig is now Clone,
            // configuration can be easily shared.
            base_config.clone().env("CLEAR_BUNDLER_CACHE", "true"),
            |context| {
                assert_contains!(context.pack_stdout, "---> Installing bundler");
                assert_contains!(context.pack_stdout, "---> Installing gems");
            },
        );
    });
}

Closes #278, GUS-W-10539929