Exempt structure tests from @covers checks

Exempt structure tests from @covers checks

@covers does not make any sense for structure tests, which either
do not cover any PHP lines (they test things like configuration or
messages), or cover lines which cannot be determined at the time
of writing the tests (e.g. they cover all classes implementing a
certain interface). Requiring @coversNothing to be manually added
for all of them is a waste of developer time.

tests/phpunit/suite.xml has forceCoversAnnotation=true so removing
the annotations will not change test coverage, these files will
still be skipped.

Change-Id: I27cb58e92341b9b1a76f109701f5bc843adbaa9b