testsuite: coverage: create kconfig option for gcov dump heap size

The heap size for serial dump of gcov data is currently
defined by MALLOC_MAX_HEAP_SIZE, which cannot be
adjusted by kconfig. This commit adds a new kconfig
option, CONFIG_COVERAGE_GCOV_HEAP_SIZE, which retains
the behavior or MALLOC_MAX_HEAP_SIZE by default.

Signed-off-by: Jason Wright <jwright@synchron.com>