[SYCL][E2E] Better control of testing `preview-mode` (#19735)
Before the PR we had two modes:
* Default, auto-detect preview support in toolchain and have individual
tests run dedicated `RUN`-lines
* `--param test-preview-mode=<anything but False>`, run entire suite in
preview mode using non-preview `RUN`-lines. Special `RUN`-lines
dedicated to preview are ignored.
This PR changes it to this:
* Only allow `test-preview-mode` to be unset or set to True/False, fatal
error on any other value.
* If True/unset behave as in two previous scenarios
* If False, only execute `RUN`-lines without preview markup
I need this for compatibility testing because preview mode isn't
backward ABI-compatible and I'm going to cherry-pick this to
`sycl-rel-6_[23]`
For trunk, I think we should just remove special `RUN`-lines and rely on
"full preview" mode job in CI, but that will be a separate PR (because I
wouldn't be able to backport such a big change).
Cherry-picks https://github.com/intel/llvm/pull/19727
