Enable cross-compiled 32bit tests on TravisCI

[slyon]

I think it should be possible to run the tests on cross-compiled 32bit binaries on the Travis linux host.
With this change I am added one single linux/amd64 build, which used the GOOS= and GOARCH= environment variables, to run the tests with i386 32bit binaries.

And indeed this test fails with very similar errors to what can be seen in the failing Debian/Ubuntu 32bit builds (#1384), i.e. Error=cannot allocate memory tracing down to /home/travis/.gimme/versions/go1.14.linux.amd64/src/runtime/asm_386.s and should thus resolve #1385

=== RUN   TestRotate
badger 2020/06/30 16:00:14 INFO: All 0 tables opened in 0s
badger 2020/06/30 16:00:14 INFO: Got compaction priority: {level:0 score:1.73 dropPrefixes:[]}
badger 2020/06/30 16:00:14 INFO: All 0 tables opened in 0s
badger 2020/06/30 16:00:14 INFO: Replaying file id: 0 at offset: 0
badger 2020/06/30 16:00:14 INFO: Replay took: 2.962µs
badger 2020/06/30 16:00:14 INFO: Got compaction priority: {level:0 score:1.73 dropPrefixes:[]}
badger 2020/06/30 16:00:14 INFO: All 0 tables opened in 0s
badger 2020/06/30 16:00:14 INFO: Replaying file id: 0 at offset: 0
badger 2020/06/30 16:00:14 INFO: Replay took: 2.936µs
badger 2020/06/30 16:00:14 INFO: Got compaction priority: {level:0 score:1.73 dropPrefixes:[]}
badger 2020/06/30 16:00:14 INFO: All 0 tables opened in 0s
badger 2020/06/30 16:00:14 INFO: Replaying file id: 0 at offset: 0
badger 2020/06/30 16:00:14 INFO: Replay took: 3.116µs
    TestRotate: rotate_test.go:91: 
        	Error Trace:	rotate_test.go:91
        	Error:      	Received unexpected error:
        	            	Map log file. Path=/tmp/badger-test181776523/000001.vlog. Error=cannot allocate memory
        	            	During db.vlog.open
        	            	github.com/dgraph-io/badger/v2/y.Wrapf
        	            		/home/travis/gopath/src/github.com/slyon/badger/y/error.go:82
        	            	github.com/dgraph-io/badger/v2.Open
        	            		/home/travis/gopath/src/github.com/slyon/badger/db.go:390
        	            	github.com/dgraph-io/badger/v2/badger/cmd.TestRotate
        	            		/home/travis/gopath/src/github.com/slyon/badger/badger/cmd/rotate_test.go:90
        	            	testing.tRunner
        	            		/home/travis/.gimme/versions/go1.14.linux.amd64/src/testing/testing.go:992
        	            	runtime.goexit
        	            		/home/travis/.gimme/versions/go1.14.linux.amd64/src/runtime/asm_386.s:1337
        	Test:       	TestRotate
--- FAIL: TestRotate (0.13s)

Reference build i386 on amd64: https://travis-ci.com/github/slyon/badger/builds/173676484
Reference build arm on arm64: https://travis-ci.com/github/slyon/badger/builds/173679498

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[jarifibrahim]

This is awesome @slyon . Thank you so much for fixing this. I'd like to fix all the failing tests in this PR itself. Is it okay if I push some fixes on this PR?

Reviewable status: 0 of 1 files reviewed, all discussions resolved (waiting on @ashish-goswami, @jarifibrahim, and @manishrjain)

[slyon]

This is awesome @slyon . Thank you so much for fixing this. I'd like to fix all the failing tests in this PR itself. Is it okay if I push some fixes on this PR?

Absolutely, that would be great @jarifibrahim ! I do not have a lot of experience with go code, otherwise I would have looked into the failures myself already.

I enabled the maintainer access to the PR branch at https://github.com/slyon/badger/tree/travis-32bit so you should have write access there. Let me know if you need any other permissions.

[jarifibrahim]

@slyon I won't be able to fix the crashes on this PR right now. I'll merge this PR and create an issue for fixing the builds. The community is pretty active in helping :)

Reviewable status: 0 of 1 files reviewed, all discussions resolved (waiting on @ashish-goswami and @manishrjain)