fmp4mux: Crank clock for the first fragment in more tests

Due to how aggregator works, it depends on how buffers are pulled whether aggregate() is called again or it is waiting for a timeout or EOS:

works:

• pad 1: 4 buffers, pad 2: 4 buffers
• aggregate ready: take all 4/4 buffers
• pad 1: 1 buffers, pad 2: 1 buffer
• aggregate ready: take all 1/1 buffers

waits:

• pad 1: 5 buffers, pad 2: 4 buffers
• aggregate ready: take all 5/4 buffers
• pad 1: 0 buffers, pad 2: 1 buffer
• aggregate not ready: waiting for timeout or EOS

Also don't manually set the clock time as that's unnecessary.

Fixes #274 (closed)