bc4382b075
[ 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