Optimize sharness tests

Some sharness tests take way more time to complete than they have to.

Results from https://ci.ipfs.team/job/IPFS/job/go-ipfs/job/PR-4959/7/testReport/sharnessLinux/ (sort by duration) / https://github.com/ipfs/go-ipfs/pull/4959

* [x] - `t0181-private-network / 12 - try connecting node in public network with priv networks` - should timeout way sooner
* [x] - `t0130-multinode` - adds a tree of 5k files multiple times, might be a bit overkill, at least of something that isn't a benchmark
* [ ] - `t0250-files-api` is huuuuge, would be good to split into some separate tests to make use of make parallelism
* [ ] - `t0260-sharding` operates on quite large directories, not sure if we can do much about it
* [ ] - `t0040-add-and-cat` does some stuff on large files, but majority of the time spent there seems to be spent in the ton of tests there, might be worth to split too