Incorrect E2E build documentation regarding workspace build

[sommerlukas]

Describe the bug
The description in sycl/test-e2e/README.md on how to setup the check-sycl-e2e target for a workspace build is incorrect.

As sycl/test-e2e/CMakeLists.txt is never included from parent CMake file, the target is not created.

To Reproduce

1. Clone this repository
2. mkdir build && cd build
3. Run configure.py as specified in sycl/test-e2e/README.md (also note the missing \ on the second line in the README):

python ../buildbot/configure.py -o . \
  --cmake-opt=-DSYCL_TEST_E2E_TARGETS="level_zero:gpu;opencl:gpu"\
  --cmake-opt=-DSYCL_E2E_TESTS_LIT_FLAGS="--param;dump_ir=True"

4. Try to run ninja check-sycl-e2e. Results in an error, as no such target is available.

Environment (please complete the following information):

• OS: Linux
• Target device and vendor: Intel OpenCL CPU (but should affect all targets)
• DPC++ version: afebb2543ccecb89f83c84b68fba7616bbab89ac

Additional context
Potential fix: Add add_subdirectory(test-e2e) to sycl/CMakeLists.txt, potentially depending on an option similar to SYCL_INCLUDE_TESTS