pkg/private: cleanup common, util, xtest packages

[matzf]

The pkg/private/common, util and xtest packages have rather fuzzy scope, and have accumulated a bit of cruft and unused or outdated functionality. Clean this up a bit:

• pkg/private/common:
  • remove unused constants
  • remove outdated error handling helpers and replace remaining use
  • remove NativeOrder and IsBigEndian: No longer needed.
    Native byte order is not often needed, but will eventually show up in standard library anyway (encoding/binary: add var NativeEndian; also x/sys/cpu.IsBigEndian golang/go#57237).
• pkg/private/util:
  • remove unused helper functionality
  • remove Checksum: only used to compute reference value in slayers test cases. Use a simpler, non-optimized implementation for this. Closes checksum: re-evaluate library #4262.
  • move RunsInDocker to private/env
  • move ASList to tools/integration
• pkg/private/xtest:
  • remove unused helpers
  • remove unused Callback and MockCallback
  • replace FailOnErr with require.NoError
  • replace AssertErrorsIs with assert.ErrorIs

There are still more things to clean up in pkg/private, in future PRs, in particular:

• common.ErrMsg should be integrated in serrors
• common.IFIDType should be removed or renamed and moved somewhere more appropriate
• Merge the remainder of util and common
• Clean up LinkType and RevInfo from pkg/private/ctrl

---

This change is 

[marcfrei]

Reviewed 59 of 59 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @oncilla)

[lukedirtwalker]

Reviewed 59 of 59 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @matzf and @oncilla)

---

tools/integration/integration.go line 1 at r1 (raw file):

// Copyright 2018 Anapaya Systemduring

revert that

Suggestion:

Systems

[matzf]

Reviewable status: 58 of 59 files reviewed, 1 unresolved discussion (waiting on @lukedirtwalker, @marcfrei, and @oncilla)

---

tools/integration/integration.go line 1 at r1 (raw file):

Previously, lukedirtwalker (Lukas Vogel) wrote…

revert that

Yes, oops! Done.

[lukedirtwalker]

Reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @oncilla)

[oncilla]

Reviewed 58 of 59 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @matzf)

---

pkg/private/util/checksum.go line 25 at r3 (raw file):

// Calculate RFC1071 checksum of supplied data chunks. If a chunk has
// an odd length, it is padded with a 0 during checksum computation.
func Checksum(srcs ...[]byte) uint16 {

Just as an FYI, this implementation was ~2 times as fast as the slayers implementation the last time I benchmarked.
But given it is not used, we don't need to keep it around.

[lukedirtwalker]

Reviewed all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @matzf)