ERT lets you define tests in the same way you define functions. You
can type ert-deftest forms in a buffer and evaluate them there
with eval-defun or compile-defun, or you can save the
file and load it, optionally byte-compiling it first.
Just like find-function is only able to find where a function
was defined if the function was loaded from a file, ERT is only able
to find where a test was defined if the test was loaded from a file.