Cache example_test results with travis

[GuessWhoSamFoo]

Since example_test.go does not change frequently, caching the tests can bring the CI times from 50s to a few seconds if there are no changes.

Signed-off-by: GuessWhoSamFoo sfoohei@gmail.com

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[tengqm]

the pro is obvious: shorter turnaround;
the con is subtle: when upstream code changes, the existing samples may break thus need fixes;
doubt: how can we explicitly request a check when new examples are added?

[bradtopol]

This looks like there are some issues that would need to be addressed before this is viable. Im going to ask @zacharysarah to keep an eye on this.

[zacharysarah]

@GuessWhoSamFoo 👋 Thanks for your patience, I'm finally back from vacation.

I like the idea, but @tengqm's concern is a valid one. Any feedback on how to check for upstream test changes?

[zparnold]

From: https://golang.org/cmd/go/#hdr-Build_and_test_caching

The build cache correctly accounts for changes to Go source files, compilers, compiler options, and so on: cleaning the cache explicitly should not be necessary in typical use.

@zacharysarah I think this is fine, if the file changes, the cache is invalidated

[zparnold]

/lgtm
(I'll wait on @zacharysarah for /approveal)

[zacharysarah]

/approve

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