This series exposes a Clock template parameter for loading_cache so that the test could use
the manual_clock rather than the lowres_clock, since relying on the latter is flaky.
In addition, the test load function is simplified to sleep some small random time and co_return the expected string,
rather than reading it from a real file, since the latter's timing might also be flaky, and it out-of-scope for this test.
Fixes#20322
* The test was flaky forever, so backport is required for all live versions.
Closesscylladb/scylladb#22064
* github.com:scylladb/scylladb:
tests: loading_cache_test: use manual_clock
utils: loading_cache: make clock_type a template parameter
test: loading_cache_test: use function-scope loader
test: loading_cache_test: simlute loader using sleep
test: lib: eventually: add sleep function param
test: lib: eventually: make *EVENTUALLY_EQUAL inline functions
9fc14f203b