[sycl-post-link][NFC] Outline legacy tests (#4844)

Moved a number of `sycl-post-link` LIT tests for specialization
constants into a separate folder: those tests are using old RT-Compiler
interface, which is only used for experimental support for spec
constants in SYCL 1.2.1.

That old version of spec constants depends on `program` class, which was
removed from SYCL 2020 specification and from our implementation.
Therefore, support for old spec constants API is not available anymore.
The API itself is still supported for backward compatibility, but it
will be completely erased at some point. This patch will simplify tests
cleanup when backward compatibility break will be done.

Author: AlexeySachkov