tests: coverage: fix the blocking on mps2_an385 coverage report

The overall code coverage report of mps2_an385 was blocked by the
tests/net/lib/coap, the error message shows "No Mem available to
continue dump". So we enlarge the gcov heap size to prevent this
situation, try to make the report can be generated at least.

Signed-off-by: Enjia Mai <enjia.mai@intel.com>