Enforces a path format for storage drivers (#819)

Requires all paths in the inmemory and filesystem drivers to begin with
a slash, and then contain only valid path components (2+ alphanumeric
characters with optional period, hyphen, and underscore separators)
delimited by slashes.

Also updates the storage driver test suites to construct paths of this
format, and causes the suite to abort if files are not cleaned up after
the test run.