[SYCL][Graph] Add documentation for layout of graph e2e tests (#13251)

Co-authored-by: Ben Tracy <[email protected]>

Author: fabiomestre

sycl/test-e2e/Graph/README.md

@@ -0,0 +1,61 @@
+## Overview
+
+The "Graph" directory contains tests for the
+[sycl_ext_oneapi_graph](../../doc/extensions/experimental/sycl_ext_oneapi_graph.asciidoc)
+extension.
+
+Many subdirectories contain a `lit.local.cfg` file. This file sets the `lit`
+option `config.required_features` to the graph aspect required to run the tests:
+
+- `aspect-ext_oneapi_limited_graph` for any test that doesn't require the
+  executable graph update feature.
+- `aspect-ext_oneapi_graph` for any test that does require the executable graph
+  update feature.
+
+## Structure
+
+Most of the tests are written in a similar manner to other `e2e` tests. The
+exception to this are tests in the `Inputs` directory which are meant to be used
+as inputs to other tests.
+
+Often, the same feature, needs to be tested for both the `Explicit`
+and `Record and Replay` APIs. To avoid code duplication, such tests are added to
+the `Inputs` folder and rely on common code from `graph_common.hpp` to construct
+the graph. The files in the `Inputs` directory are not run directly by `lit`.
+Instead, that source is included by tests in the `Explicit` and `RecordReplay`
+directories. These tests also define `GRAPH_E2E_EXPLICIT`
+or `GRAPH_E2E_RECORD_REPLAY` respectively to choose which API is used by the
+common code.
+
+The other directories are used to group similar tests together. Tests that
+require a specific `aspect` are also grouped together in order to use the
+`lit.local.cfg` file. Directories might themselves contain subdirectories named 
+`Explicit` and `RecordReplay` if they make use of the framework described above.
+
+In addition, in order to help identify specific tests, the matching files
+in `Explicit`, `RecordReplay` and `Inputs` folders should have identical names.
+
+## Test Execution
+
+Tests might be run multiple times using different options. The most commonly used
+options are:
+
+- `l0_leak_check`: `lit` substitution which, on the `level-zero` backend, enables
+checks for memory leaks caused by mismatched number of calls to memory
+allocation / release APIs in `Unified Runtime`.
+- `SYCL_PI_LEVEL_ZERO_USE_IMMEDIATE_COMMANDLISTS`: Environment variable which, 
+on the `level-zero` backend, enables or disables
+[immediate command-lists](https://spec.oneapi.io/level-zero/latest/core/PROG.html#low-latency-immediate-command-lists).
+Without this option, depending on the hardware, immediate command-lists might be
+enabled or disabled by default.
+
+Tests might be run multiple times using different combination of the options 
+described above. Most tests do the following:
+
+1. A default run which runs for all backends. 
+2. On the `level-zero` backend only, tests for leaks and forcefully **disables**
+immediate command-lists in order to test this codepath on hardware that enables
+immediate command-lists by default.
+3. On the `level-zero` backend only, tests for leaks and forcefully **enables**
+immediate command-lists in order to test this codepath on hardware that disables
+immediate command-lists by default.