travis: also test go-fuzz with Go tip (including because oss-fuzz is using tip)

[thepudds]

In travis, add tests for Go tip on linux and osx.

In tip and Go 1.13, GO111MODULE defaults to on (whereas it defaults to auto in 1.11 and 1.12).

However, travis explicitly sets GO111MODULE=auto. Therefore, when testing tip, we explicitly set GO111MODULE=on to give normal tip / 1.13 behavior.

Note: it does not work just to use env: GO111MODULE=on because then go get stops working (perhaps because go-fuzz is not yet a module). Instead, we use SET_GO111MODULE in the travis configuration, and then check that to know whether or not to do export GO111MODULE=on just before invoking go-fuzz-build and go-fuzz.

Separately, pull request #237 was merged, but that current fix is probably incomplete for disabling modules. With these changes, travis passes with 1.11 and 1.12, but currently fails on tip with:

$ go-fuzz-build
could not resolve package ".": go [list -e -json -compiled=false -test=false -export=false -deps=false -find=true -tags gofuzz -- .]: exit status 1: go: cannot find main module, but found .git/config in /home/travis/gopath/src/github.com/dvyukov/go-fuzz
	to create a module there, run:
	cd .. && go mod init

Updates #237
Updates google/oss-fuzz#2188

[josharian]

Thanks, @thepudds. This LGTM, but I'll let @dvyukov take a look before merging.

Out of curiosity, does this pass with #240 included?

(Also, out of curiosity, why don't you dive into the code yourself? You have useful and insightful comments.)

[thepudds]

Out of curiosity, does this pass with #240 included?

Hi @josharian, I need to run, but I can try that later or maybe tomorrow. (I was going to say I thought go-fuzz would need to be updated too... but you were too fast and now looks like you updated go-fuzz too ;-)

[thepudds]

And sorry, in my fork I was juggling branches trying to see if os.Setenv would work to disable modules (and trying to test it with travis), but I see now that I messed up this PR by later accidentally adding one of the os.Setenv changes. My mistake.

I can try to correct that tomorrow as well.

[thepudds]

OK, this PR is back to just the one expected changed file and ready for any additional review. Sorry about that.

And good news: as expected or hoped, with #240 now merged to master, this PR now passes travis (whereas prior to #240 being merged, travis would fail this PR with Go tip due to modules being enabled by default on tip / 1.13).