runtime: panic index in FindScavengeCandidate tests

[mknyszek]

The builder linux-mips64le-mengzhuo is failing at its latest change with a panic index in scavenge tests for the new page allocator.

https://build.golang.org/log/ad51dd54c3603d92b85a219fbbba9c4d21794f3a

The problem is that it has a larger huge page size than any other system I've tested on thus far (32 MiB) so the fix is to just handle those cases correctly, or to no longer handle those cases at all.

I think it's OK to not handle them at all in the short term because our allocator is not huge-page aware, so the likelihood that 32 aligned MiB is going to be totally free and unscavenged is very low. The exception of course is a very large transient heap spike, so this should absolutely get fixed.

I'm going to push a short-term fix and work on a slightly bigger fix.

@bcmills @aclements @randall77 @cherrymui

[gopherbot]

Change https://golang.org/cl/205937 mentions this issue: runtime: define maximum supported physical page and huge page sizes