Create test for pinning one million files

[whyrusleeping]

To help detect and prevent issues like #3273, we should create a test that doesnt necessarily have to run in CI, but that we can run whenever affected code is touched.

Simply, all we need to do is create 1,000,000 unique files, and add each of them. After each add we should check that we have the correct number of pinned files, and at the end, make sure that the entire pinset looks correct.

We have smaller tests like this, but never ramped up the numbers this high due to perf issues with ipfs itself.

[whyrusleeping]

This exists as test/sharness/x0601-pin-fail-test.sh