Limit logging for verbose system tests

Limit logging for verbose system tests

The system test framework starts all named instances with the "-d 99"
command line option (unless it is overridden by a named.args file in a
given instance's working directory).  This causes a lot of log messages
to be written to named.run files - currently over 5 million lines for a
single test suite run.  While debugging information preserved in the log
files is essential for troubleshooting intermittent test failures, some
system tests involve sending hundreds or even thousands of queries,
which causes the relevant log files to explode in size.  When multiple
tests (or even multiple test suites) are run in parallel, excessive
logging contributes considerably to the I/O load on the test host,
increasing the odds of intermittent test failures getting triggered.

Decrease the debug level for the seven most verbose named instances:

  - use "-d 3" for ns2 in the "cacheclean" system test (it is the lowest
    logging level at which the test still passes without the need to
    apply any changes to tests.sh),

  - use "-d 1" for the other six named instances.

This roughly halves the number of lines logged by each test suite run
while still leaving enough information in the logs to allow at least
basic troubleshooting in case of test failures.

This approach was chosen as it results in a greater decrease in the
number of lines logged than running all named instances with "-d 3",
without causing any test failures.

(cherry picked from commit 4a8d404876f88e6f9f030c4b80db77cadfb783a8)