coverage: do not dump coverage data by default

Only dump data when we are interested in the analysing coverage. By
default just collect the data.

CONFIG_COVERAGE_DUMP is used to control this behaviour.

This will help speed up sanitycheck and will avoid lots of noise in the
log when some tests with coverage enabled failed. Dumping data to
console is also suspected to be one of the reason why qemu hangs in CI.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>