OK, let's examine "if they could".
If it was just a systems test, they could launch just off the China coast.
Unannounced, in international waters off the LA coast would be a political test. Remember, a nation can launch unarmed missiles from international waters, as long as they are not trajectoried over another nation without permission. This would upset the US military to what purpose? Drive American influence out of California? Zer0 and the Rats have already done that for free.
China will take confrontational actions in the South China Sea, around Taiwan, the Yellow Sea, and Korea, but here it is clearly expected, within policy, and doesn't piss off anyone important.
China is becoming strong militarily, and intends to become much stronger, but she doesn't intend to move against US. She has already defeated us in business, and intends to continue doing so.
As far as a test of Zer0's weakness, China already knows he is weak, they don't need to test.
The only testing China will do is regarding removing US influence in the WESTERN Pacific.