scripts: twisterlib: coverage: exclude `LOG_*` branches

Disable the branch coverage calculations on the `LOG_*` family of macros.
Branch misses are due to the implementation of `Z_LOG2` and cannot be
reasonably covered in library code. The internal paths through the
`Z_LOG2` macro are not of interest when inspecting files that happen to
use the Zephyr logging API.

Signed-off-by: Jordan Yates <jordan@embeint.com>