[Tests] Improve tests about warning-free SYCL headers

### Is your feature request related to a problem? Please describe

This is prompted by discussion we had in #15064: we already have some testing to check that SYCL headers are warning-free, but it can always be improved. In particular, we allow to use a 3rd party compiler to build host part of SYCL applications and in that phase/mode SYCL headers are not considered to be system headers, so any warnings coming out of them may break `-Werror` builds or just annoy users polluting their build logs.

### Describe the solution you would like

As a first step, we should copy existing `warnings.cpp` test and compile it using `gcc` instead of `clang++`:

https://github.com/intel/llvm/blob/b8615ffe6065992147b41359bcb4646e22f7a432/sycl/test/warnings/warnings.cpp#L1-L3

Going forward, this (and existing) tests could be further expanded, but having a test with `gcc` would be already a good start.

### Describe alternatives you have considered

_No response_

### Additional context

_No response_