boards: tlsr9518adk80d: Disable testing due to invalid linker script

This commit disables testing for the `tlsr9518adk80d` platform because
it contains an invalid linker script (overlapping memory region) and
cannot be compiled using the current version of the Zephyr SDK (0.15.0
at the time of making this change).

Revert this commit to re-enable testing for this platform once the
GitHub issue #49036 is fixed.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>