Drop these stale build instructions.

Don't try to keep README.txt in sync with the web page; just point people to the Web.
Just compile 'helper' functions into each test.

It simplifies things for llvm-lit if we have one source file per test (or at least it will once I commit the test_exec_root change!).
Set config.test_exec_root when testing from CMake.

When running "ninja test" (or "make test" if you're into that sort of thing), don't pollute the source directory with test outputs.
Refactor common test information into /lit.cfg.

No need to duplicate this test setup information across libtesla/test and tesla/test.