[llvm] [libsycl] Add lit configuration files and basic test (PR #177407)

[Kseniya Tikhomirova via llvm-commits]

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+## Getting Started
+
+This directory contains `libsycl` tests distributed in subdirectories based on
+testing scope. `libsycl` uses LIT to configure and run its tests.
+
+Please see the [Lit Command Guide](https://llvm.org/docs/CommandGuide/lit.html)
+for more information about LIT.
+
+## Prerequisites
+
+* Target runtime(s) to execute tests on devices.
+  TODO: add link to liboffload instruction once they add it.
+* Compiler & libsycl. Can be built following these
+  [instructions](/libsycl/docs/index.rst).
+
+## Run the tests
+
+`libsycl` is integrated via `LLVM_ENABLE_RUNTIMES` and is not visible as top
+level target. Same is applicable to tests. To run `check-sycl` tests you need
+to prefix `<build>/runtimes/runtimes-bins/` to the paths of all tests.
+For example, to run all the libsycl tests you can do:
+```bash
+<build>/bin/llvm-lit <build>/runtimes/runtimes-bins/libsycl/test
+```
+
+To run individual test, use the path to it instead.
----------------
KseniyaTikhomirova wrote:

https://github.com/llvm/llvm-project/pull/177407/commits/89e602ad6f159cd09d4bef5a8fcefc343d38a9cc

https://github.com/llvm/llvm-project/pull/177407