KCIDB: v5.3 - test input files

Hello, everyone involved with sending results to KCIDB,

The v5.2 schema adding issue categories has been merged, and we're testing the
already-implemented DB support for it.

Let's now move onto the next change in v5.3: adding support for
specifying test input files (an `input_files` field):

https://github.com/kernelci/kcidb-io/pull/99/commits/07cc1885a1ecae6f25287b61b58ecb4f70a92e63

We already have `input_files` in builds, and `output_files` in both builds and
tests. So this change is, again, quite straightforward. Simone Tollardo, who
works on RISC-V kernel, asked for this (and did the schema change!) to support
specifying the rootfs which their tests executed on. Something like this:

{
    "version": {
        "major": 5,
        "minor": 2
    },
    "tests": [
        {
            "id": "riscv:ab266247c76746e",
            "origin": "riscv",
            "build_id": "maestro:27a3f84da7898ec9f4f1be8e4e043",
            "path": "super-test",
            "input_files": [
                {
                    "name": "rootfs.img",
                    "url": "https://github.com/riscv/kernel/commit/"
                           "f89927a3f84da7898ec9f4f1be8e4e0433db7879/"
                           "checks/39474543369/rootfs.img"
                }
            ]
            "status": "PASS",
        }
    ]
}

NOTE: This is based on the assumption that there could be more than one rootfs
      per build, so the build reference is not enough. Plus it's likely there
      could be other input files for tests, such as configuration, or sample
      data. Please respond, if you disagree.

As the change is minor and backwards-compatible, I'd like to set the feedback
deadline to the middle of next week, Wed, Apr 2. If there are no objections by
that time, I'll merge this change to the schema and database support on that
day.

Thank you!
Nick