[SYCL][Doc] Add test plan for sycl_ext_intel_kernel_queries extension

sycl/unittests/Extensions/KernelQueries/test_plan.md

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+# Test plan for [`sycl ext intel kernel queries`][spec-link] extension
+
+## Testing scope
+
+### Device coverage
+
+The unit tests should be launched on every supported device configuration we 
+have.
+
+### Type coverage
+
+Each query is templated on a single template type argument `Param`
+with some queries being template overloads of each other.
+Each query has a restriction on the value of `Param` defined
+in the relevant section of the spec.
+Each `Param` defines a related sycl aspect that signals whether the device
+supports it.
+
+Param must be one of the following types defined in 
+`sycl::ext::intel::info::kernel_device_specific` namespace:
+- `spill_memory_size`
+
+The tests should cover all of these types.
+
+## Tests
+
+### Unit tests
+
+#### Interface tests
+
+These tests are intended to check that all classes and methods defined by the
+extension have correct implementation, i.e.: right signatures, right return
+types, all necessary constraints are checked/enforced, etc.
+
+These tests should check the following:
+
+- that each query is not available when the template argument `Param` has
+  a value different than the one in the spec.
+- that each query can be called with the appropriate value for `Param` and the
+  appropriate argument types as defined by its signature.
+- the return types of all queries match the spec.
+
+Tests in this category may not perform some useful actions to exercise the
+extension functionality in full, but instead they are focused on making sure
+that all APIs are consistent with respect to other APIs.
+
+#### Check behavior in the case of unsupported aspects
+
+Verigy that a synchronous `exception` with the error code
+`errc::feature_not_supported` is thrown if an aspect is not supported by the
+device.
+
+[spec-link]: https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/supported/sycl_ext_intel_kernel_queries.asciidoc