more fine grained control over logging from tracers

[ivoysey]

right now, the tracers emit the same logs in every test. this is going to be a problem in the future at least because, when we do performance testing, we'll want to remove those commands to save gas so at the least we should be able to turn logs off. i added a boolean to mark where that will happen in 54fde19 but it should probably be a commandline flag or test option or both.

more generally: the same logs always get emitted, so if i change the code for it i often have to go back and change old tests. that's been fine so far but won't fly forever.

one approach is to define a JSON field of logging shim code in the test file that gets inserted into the tracers per-test, or a file name to read and get inserted so that at least you don't have to recompile the compiler every time you want to emit different logs.