Tests do not fully stress the RMII Mac running at 75MHz (design goal) thread speed

[ed-xmos]

The tests use multiple tasks and some filler threads however spare MIPS will be liberated meaning MAC threads will likely see more than 75MHz.

We should add a specific test, especially since we don't support XTA any more.

Without setting fast mode in parts of the rmii_mac the easiest way to constrain is to run the whole tile slower (eg. 370MHz/5 = 74MHz). We get linker warnings about ref clock but it should be OK just for one test,

It needn't be too in-depth - just test various tail lengths for Rx/Tx for 1b/4b.