[NFC][Doc] Update test-e2e/readme.md (#16967)

Add new sections to the table of contents and make them subsections.

Author: KornevNikita

sycl/test-e2e/README.md

@@ -9,6 +9,8 @@
 * [Creating or modifying tests](#creating-or-modifying-tests)
   * [LIT feature checks](#lit-feature-checks)
   * [llvm-lit parameters](#llvm-lit-parameters)
+  * [Marking tests as expected to fail](#marking-tests-as-expected-to-fail)
+  * [Marking tests as unsupported](#marking-tests-as-unsupported)
 * [SYCL core header file](#sycl-core-header-file)
 
 ## Overview
@@ -301,23 +303,6 @@ llvm-lit --param dpcpp_compiler=path/to/clang++ --param dump_ir=True \
          SYCL/External/RSBench
 ```
 
-### SYCL core header file
-
-While SYCL specification dictates that the only user-visible interface is
-`<sycl/sycl.hpp>` header file we found out that as the implementation and
-multiple extensions grew, the compile time was getting worse and worse,
-negatively affecting our CI turnaround time. As such, we decided to use
-finer-grained includes for the end-to-end tests used in this project (under
-`sycl/test-e2e/` folder).
-
-At this moment all the tests have been updated to include a limited set of
-headers only. However, the work of eliminating unnecessary dependencies between
-implementation header files is still in progress and the final set of these
-"fine-grained" includes that might be officially documented and suggested for
-customers to use isn't determined yet. **Until then, code outside of this
-project must keep using `<sycl/sycl.hpp>` provided by the SYCL2020
-specification.**
-
 ### Marking tests as expected to fail
 
 Every test should be written in a way that it is either passed, or it is skipped
@@ -382,6 +367,23 @@ non-standard mechanism. Use `UNSUPPORTED: true` instead, we track `UNSUPPORTED`
 tests using the mechanism described above. Otherwise the test risks remaining
 untraceable.
 
+### SYCL core header file
+
+While SYCL specification dictates that the only user-visible interface is
+`<sycl/sycl.hpp>` header file we found out that as the implementation and
+multiple extensions grew, the compile time was getting worse and worse,
+negatively affecting our CI turnaround time. As such, we decided to use
+finer-grained includes for the end-to-end tests used in this project (under
+`sycl/test-e2e/` folder).
+
+At this moment all the tests have been updated to include a limited set of
+headers only. However, the work of eliminating unnecessary dependencies between
+implementation header files is still in progress and the final set of these
+"fine-grained" includes that might be officially documented and suggested for
+customers to use isn't determined yet. **Until then, code outside of this
+project must keep using `<sycl/sycl.hpp>` provided by the SYCL2020
+specification.**
+
 ### Compiling and executing tests on separate systems
 
 The execution of e2e tests can be separated into compilation and execution