Update tester's docs according to new hierarchy

test/README.md

 only the matching ones.
 
 Tests can be written in any scripting language--they just need to be
-put into test/scripts directory, equipped with proper shebang line and
-marked as executable.
+put into test/tests directory, where each test consists of directory
+containing an executable file TF_RUN along with all data needed for the
+execution.
 
 
 Naming