Clear the symbol map between each test.

This way we could, for example, test function replacement properly.

Actually slows down tests because it spends more time correctly
determining the function boundaries.
Spend a bit less time in Read_Instruction on load.
Speed up MIPSGetInstruction() by 35%.

It's not that hot, but gets called a lot when loading modules / running
tests.  Well, and also interpreter.

Some of the previous checks were just not necessary (we never have Rese in
the tables, for example, and let's just not.)