Document get_partition_subsize and ss_size_lookup.

Add comments for the get_partition_subsize and get_plane_block_size
functions and the related lookup tables.

Add test/blockd_test.cc for the unit tests for av1/common/blockd.{h,c}.

BUG=aomedia:2044

Change-Id: I75a771369c2352afe2e511429235708a6072ed53
diff --git a/test/test.cmake b/test/test.cmake
index 57a69fb..d53b6e9 100644
--- a/test/test.cmake
+++ b/test/test.cmake
@@ -26,6 +26,7 @@
 list(APPEND AOM_UNIT_TEST_COMMON_SOURCES
             "${AOM_ROOT}/test/acm_random.h"
             "${AOM_ROOT}/test/aom_integer_test.cc"
+            "${AOM_ROOT}/test/blockd_test.cc"
             "${AOM_ROOT}/test/clear_system_state.h"
             "${AOM_ROOT}/test/codec_factory.h"
             "${AOM_ROOT}/test/decode_test_driver.cc"