test(spanner): fix errors not reporting when emulator test passes (#4451

)

Emulator tests were overwritting the results of the main test
run which can cause errors that occured in main test run not
to be reported when the emulator tests pass. Now emulator tests
append to the test log.

Updates: #4450

spanner/emulator_test.sh

@@ -43,4 +43,4 @@ function cleanup() {
 }
 trap cleanup EXIT
 
-go test -v -timeout 10m ./... -run '^TestIntegration_' 2>&1 | tee sponge_log.log
+go test -v -timeout 10m ./... -run '^TestIntegration_' 2>&1 | tee -a sponge_log.log