Split up the system/core/ TEST_MAPPING.

A lot of things had moved out of system/core/ without their TEST_MAPPING
entries having gone with them, reducing the amount of presubmit coverage
for those things.

In order to reduce the likelihood of that happening again, I've pushed
all that remained in the system/core/ TEST_MAPPING down into the
individual subdirectories.

Test: treehugger
Change-Id: Ib75d65f9200fa64ae1552471da6fbe5b7023cf94