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

[Nick Sarnie 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
----------------
sarnex wrote:

```suggestion
`libsycl` is integrated via `LLVM_ENABLE_RUNTIMES` and is not visible as a top
```

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