[ Merge of http://go/wvgerrit/209590 ]
The filestore unit tests have not been updated in a while, and
contained several test statements which could crash the test
when failed (accessing elements in a vector or characters in a
string without proper size checks). Other parts of the tests
had non-obvious purposes without detailed knowledge of how the
file system works on the different platforms.
Significant parts of the tests have been refactored to include
better checking and to add error messages to explain the
expectations. Several of the tests have been documented, and the
FileSystem header has been updated to explain what the API does.
Bug: 376533901
Test: file_store_unittest on Oriole
Change-Id: I5af9fd2a2ed01aa6186026761c9e0814604ec610
bc4382b075