net_test: fix exit code.

Exit code should be:
0 on success
non-zero on failure

Bug: 36510518
Test: m -j32 kernel_net_tests && adb sync data && adb shell
/data/nativetest64/kernel_net_tests/kernel_net_tests
Change-Id: Ief32d887828adb4a7e63454b79aca7d24a0a0567