ofs: Migrate TextContext to `libtest-mimic`

ofs currently conducts tests by creating a new context for each one, which increases the likelihood of encountering mount/unmount related issues. Perhaps we could switch to using `libtest-mimic` to gain more control over this process, such as setting up before all tests and shutting down afterwards.

Start by locating the ofs tests that create a TextContext for each test and read how their mount and unmount lifecycle is managed. Review the libtest-mimic approach and determine how setup before all tests and shutdown afterwards should fit that lifecycle. Done means ofs uses the migrated test harness with shared lifecycle control and avoids the reported mount/unmount issues.

Written by the indexing model from the issue text.