Don't set the DEBUG flag any more.

We now (correctly) rely entirely on -D NDEBUG, so stop setting -D DEBUG.
string(NULL) != string("").
Rename .tesla file tests to .tesla extensions.

To make this work, we have to abuse the C preprocessor more explicitly, rather than running TESLA files through the Clang driver.
Trivial: Vim syntax for lit.cfg files.
Test instrumentation of static functions.