SYCL: fix typo in a test and clean-up non-standard-compliant code

SYCL: fix typo in a test and clean-up non-standard-compliant code

Discovered when trying to compile with hipSYCL. Although not an issue
for Intel compiler, better keep things clean.

The change of `write` to `read` in the test code is intended. Not sure
how it worked before, but it should be `read` there.

Also fixed namespace qualifiers, implicit ctors.