Conditionally test fallback/secondary queue via SYCL_CTS_ENABLE_DEPRECATED_FEATURES_TESTS

This issue was opened as a result of KhronosGroup/SYCL-Docs#615.

The CTS has an existing option `SYCL_CTS_ENABLE_DEPRECATED_FEATURES_TESTS` which enables / disables testing of deprecated features. Because KhronosGroup/SYCL-Docs#615 deprecates some features in SYCL-Next, we should conditionally test these features in SYCL-Next only when that option is enabled.

This opens a general question about how to handle `SYCL_CTS_ENABLE_DEPRECATED_FEATURES_TESTS` with a unified CTS. Some features (like the secondary queue) are deprecated only in SYCL-Next. Presumably, we want these features to be unconditionally tested when running the CTS in SYCL 2020 mode. However, when running the CTS in SYL-Next mode, they should be tested only when `SYCL_CTS_ENABLE_DEPRECATED_FEATURES_TESTS` is enabled.

Alternatively, we could decide that `SYCL_CTS_ENABLE_DEPRECATED_FEATURES_TESTS` is no longer needed, and we could remove that option from the CTS. Since deprecated features are still required by the specification, this option can't be used for conformance submissions anyway.

Start by resolving whether deprecated-feature testing should remain controlled by SYCL_CTS_ENABLE_DEPRECATED_FEATURES_TESTS, then inspect where that option, SYCL 2020 mode, SYCL-Next mode, and secondary-queue tests are handled. Done means the chosen policy is documented in the implementation and secondary-queue coverage follows it consistently in both modes.

Written by the indexing model from the issue text.