The compaction manager's empty constructor is supposed to be invoked
only in testing environment, however, it is easy to invoke it by mistake
from production code.
Here we add a more verbose constructor and making the default compaction
private, the verbose compiler need to be invoked with a tag
for_testing_tag, this will ensure that this constructor will be invoked
only when intended.
The unit tests were changed according to this new paradigm.
Tests: unit (dev)
Signed-off-by: Eliran Sinvani <eliransin@scylladb.com>