[SYCL][DOC] Document test-related processes (#2760)

* Adding documentation for tests development

Author: vladimirlaz

CONTRIBUTING.md

@@ -20,6 +20,11 @@ is strongly encouraged that you submit the patch to https://llvm.org/ directly.
 See [LLVM contribution guidelines](https://llvm.org/docs/Contributing.html)
 for more information.
 
+**NB**: A change in compiler and runtime should be accompanied with
+corresponding test changes.
+See [Test DPC++ toolchain](sycl/doc/GetStartedGuide.md#test-dpc-toolchain)
+section of Get Started guide for more information.
+
 **Note (October, 2020)**: DPC++ runtime and compiler ABI is currently in frozen
 state. This means that no ABI-breaking changes will be accepted by default.
 Project maintainers may still approve breaking changes in some cases. Please,

sycl/doc/GetStartedGuide.md

@@ -280,7 +280,40 @@ command:
 
 ### Test DPC++ toolchain
 
-#### Run regression tests
+Every product change should be accompanied with corresponding test modification
+(adding new test(s), extending, removing or modifying existing test(s)).
+
+There are 3 types of tests which are used for DPC++ toolchain validation:
+* DPC++ in-tree LIT tests including [check-llvm](../../llvm/test),
+[check-clang](../../clang/test), [check-llvm-spirv](../../llvm-spirv/test) and
+[check-sycl](../../sycl/test) targets stored in this repository. These tests
+should not have hardware (e.g. GPU, FPGA, etc.) or external software
+dependencies (e.g. OpenCL, Level Zero, CUDA runtimes). All tests not following
+this approach should be moved to DPC++ end-to-end or SYCL-CTS tests.
+However, the tests for a feature under active development requiring atomic
+change for tests and product can be put to
+[sycl/test/on-device](../../sycl/test/on-device) temporarily. It is developer
+responsibility to move the tests to DPC++ E2E test suite or SYCL-CTS once
+the feature is stabilized.
+
+* DPC++ end-to-end (E2E) tests which are extension to
+[LLVM\* test suite](https://github.com/intel/llvm-test-suite/tree/intel/SYCL).
+A test which requires full stack including backend runtimes (e.g. OpenCL,
+Level Zero or CUDA) should be put to DPC++ E2E test suite following
+[CONTRIBUTING](https://github.com/intel/llvm-test-suite/blob/intel/CONTRIBUTING.md).
+
+* SYCL-CTS are official 
+[Khronos\* SYCL\* conformance tests](https://github.com/KhronosGroup/SYCL-CTS).
+They verify SYCL specification compatibility. All implementation details or
+extensions are out of scope for the tests. If SYCL specification has changed
+(SYCL CTS tests conflict with recent version of SYCL specification) or change
+is required in the way the tests are built with DPC++ compiler (defined in
+[FindIntel_SYCL](https://github.com/KhronosGroup/SYCL-CTS/blob/SYCL-1.2.1/master/cmake/FindIntel_SYCL.cmake))
+pull request should be created under
+[KhronosGroup/SYCL-CTS](https://github.com/KhronosGroup/SYCL-CTS) with required
+patch.
+
+#### Run in-tree LIT tests
 
 To verify that built DPC++ toolchain is working correctly, run:
 
@@ -302,6 +335,11 @@ skipped.
 If CUDA support has been built, it is tested only if there are CUDA devices
 available.
 
+#### Run DPC++ E2E test suite
+
+Follow instructions from the link below to build and run tests:
+[README](https://github.com/intel/llvm-test-suite/tree/intel/SYCL#execution)
+
 #### Run Khronos\* SYCL\* conformance test suite (optional)
 
 Khronos\* SYCL\* conformance test suite (CTS) is intended to validate

sycl/test/on-device/README.md

@@ -0,0 +1,13 @@
+# DPC++ end-to-end tests for features under development
+
+We require to have in-tree LIT tests independent from HW (e.g. GPU,
+FPGA, etc) and external software (e.g OpenCL, Level Zero, CUDA runtimes, etc).
+
+However, it might be helpful to have E2E tests stored in-tree for features
+under active development if atomic change is required for the test and product.
+This directory can contain such tests temporarily.
+
+It is developer responsibility to move the tests from this directory to
+[DPC++ E2E test suite](https://github.com/intel/llvm-test-suite/tree/intel/SYCL)
+or [KhronosGroup/SYCL-CTS](https://github.com/KhronosGroup/SYCL-CTS) once the
+feature is stabilized.