dashboard/app: allow specifying more parameters when testing patches

[xairy]

Current it's only possible to specify kernel tree + commit id and a patch to the #syz test command. There's no way to specify:

• kernel .config,
• reproducer,
• syzkaller version.

For those parameters, syzbot currently uses the values that correspond to the newest reproducer available on the dashboard at the time of testing. This can lead to false-positive Tested-by responses, when the bug is not reproducible with the provided combination of all parameters due to breaking changes in one of the components (kernel or syzkaller).

We could allow specifying more parameters for the #syz test command. The kernel .config and reproducer can be detected as attachment files named e.g. .config and syz. syzkaller commit id can be provided as an optional third argument to #syz test.

Related to #1609.

[dvyukov]

FTR, Alan also asked for ability to supply a reproducer for a bug w/o a reproducer:
https://lore.kernel.org/lkml/20210415205957.GA19917@rowland.harvard.edu/