test.bash: add -failfast flag

Rather than waiting for all tests for all Go versions to finish
only to find that there was some minor breakage,
fail fast so that we stop wasting time.

Change-Id: Ie255ceb5ac2cbdc598a074c6da281f5e49eb1326
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189019
Reviewed-by: Damien Neil <dneil@google.com>

test.bash

@@ -4,5 +4,5 @@
 # license that can be found in the LICENSE file.
 
 cd "$(git rev-parse --show-toplevel)"
-go test -v -mod=vendor -timeout=60m -count=1 integration_test.go "$@"
+go test -v -mod=vendor -timeout=60m -count=1 integration_test.go -failfast "$@"
 exit $?