Add test dependencies flag to go get

[alexmcroberts]

When running this build on CodeBuild, test dependencies were not fetched, which led to PRE_BUILD errors as follows:

main_test.go:9:2: cannot find package "github.com/stretchr/testify/assert" in any of: /usr/local/go/src/github.com/stretchr/testify/assert (from $GOROOT) /go/src/github.com/stretchr/testify/assert (from $GOPATH) /codebuild/output/src283397697/src/github.com/stretchr/testify/assert

By adding the -t flag, dependencies for tests are also fetched. More information on flags available for go get are available in the documentation for the get package

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