net_test: return non-zero exit code on failure.

Otherwise, our test infra can't distinguish between pass and fail.

Test: m -j32 kernel_net_tests && adb sync data && adb shell
/data/nativetest64/kernel_net_tests/kernel_net_tests
Change-Id: Icafe27db80f98ce314e55e2f5b0c75dff786090d