Enable cross-compiled 32bit tests on TravisCI (#1392)

This PR enables 32-bit cross compilation on Travis CI. There have been multiple
reports of badger tests failing on 32-bit machines and a 32 bit CI would be
very useful.

This PR creates a new Travis CI job which builds and runs tests on a 32-bit machine.

Fixes #1385

.travis.yml

@@ -24,13 +24,27 @@ jobs:
       env: GOARCH=386
     - go: tip
       env: GOARCH=386
+  include:
+    # Define one extra linux build, which we use to run cross
+    # compiled 32 bit tests
+    - os: linux
+      arch: arm64
+      go: "1.14"
+      env: go_32=yes
 
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-script:
-- go test -v ./...
-# Another round of tests after turning off mmap.
-- go test -v -vlog_mmap=false github.com/dgraph-io/badger
+script: >-
+    if [ $TRAVIS_OS_NAME = "linux" ] && [ $go_32 ]; then
+      uname -a
+      GOOS=linux GOARCH=arm go test -v ./...
+      # Another round of tests after turning off mmap.
+      GOOS=linux GOARCH=arm go test -v -vlog_mmap=false github.com/dgraph-io/badger
+    else
+      go test -v ./...
+      # Another round of tests after turning off mmap.
+      go test -v -vlog_mmap=false github.com/dgraph-io/badger
+    fi