Auto merge of #81535 - nikic:update-test-various, r=sanxiyn

Update test-various to Ubuntu 20.04

The test command-setgroups.rs is adjusted to skip on musl, where `sysconf(_SC_NGROUPS_MAX)` always returns a dummy value of 32, even though the actual value is 65536. I'm not sure why this only became a problem now, as the information I found indicates that this value changed in kernel version 2.6.4, which is ages ago.

I'm a bit unsure whether this one will go through, because I locally also saw a failure in std-backtrace.rs which went away on subsequent runs, and also had port assignment failures, but I think those might be on my side. I'm kind of curious how the code in https://github.com/rust-lang/rust/blob/b122908617436af187252572ed5db96850551380/library/std/src/net/test.rs#L43-L56 is supposed to work, as the directory names it checks don't seem to appear anywhere else.

r? `@Mark-Simulacrum`

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