system-info, runtime: Factor out SrtRuntime data structure
system-info: Factor out parsing of SrtRuntime from JSON report
system-info: Factor out discovery of Steam Runtime information
system-info: Check whether the Steam Runtime is on an unsupported filesystem
The preferred way to install all versions of the Steam Runtime, both LD_LIBRARY_PATH and container, is on a fully-featured local Unix filesystem such as ext4, btrfs or xfs. If that is not how it's installed, we should flag that in system-info reports so that we can treat it as a possible root cause for user-visible issues: as with many of our issue flags, it isn't necessarily a problem, but it's a potential problem.
Ignore these issue flags during unit testing, because the filesystem we're running on during unit testing isn't under our control.