golden example tests

The tests in this package are used to:

1. that gazelle runs properly (generates correct output from BUILD.in to BUILD.out)
2. that the build artifacts can actually be built (bazel build //...).
3. Generate documentation.

//example/golden:golden_test runs the gazelle tests. Each subdirectory of testdata/* represents a separate test. The BUILD.in file is used as the base, and BUILD.out is expected to be generated. If there is a diff against the actual BUILD.out, the test fails.

The individual gazelle_testdata_example tests package up specific examples and generate a go_bazel_test for the example. So for //example/golden:java, a //example/golden:java_test will run a test that actually tries to build everything within the workspace based on the contents of BUILD.out.