fix go-fuzz failing at Go tip, plus get testing passing again

[thepudds]

This PR hopefully addresses a couple of go-fuzz issues, while also updating tests to handle changes in cmd/go. Travis has been failing on master for go-fuzz for ~5 different reasons for several months, but with these changes, travis is now passing again on my fork.

• go-fuzz-build:

  • handle abi_amd64.h (new in go1.17) for #322.
  • do not doubly preserve '//go:build' comments for #313.

• testscripts:

  • cmd/go now defaults to -mod=readonly, so explicitly add go-fuzz-dep via 'go get'.
  • cmd/go now requires a valid go.sum entry, so use 'go get' rather than go.mod for go-fuzz-dep.
  • '-h' no longer causes non-zero status code as of Go 1.15 or so.

• travis

  • golang/go#44487 was likely causing go-fuzz travis tests to fail with tip for several months or so due to cmd/go panic, likely due to the // +build in slides/crash.go and elsewhere.
  • update the tested versions of Go.
  • re-order OS/versions (partly because travis-ci.com is seemingly slower than travis-ci.org)

Fixes #313 Fixes #322

[thepudds]

Currently running travis tests:

https://travis-ci.org/github/dvyukov/go-fuzz/builds/773061221