Change "test cases" to "test files"

Prior to this change, we used test case and test file interchangeably,
but in an attempt to clear up wording, test case refers to the tests the
autograder applies, and test file refers to the tests the students
submit.

Makefile

@@ -143,23 +143,23 @@ clean:
 	rm -Rf *.dSYM
 
 # make partialsubmit.tar.gz - cleans, creates tarball
-# omitting test cases
+# omitting test files
 PARTIAL_SUBMITFILES=$(filter-out $(TESTSOURCES), \
                       $(wildcard Makefile *.h *.hpp *.cpp))
 $(PARTIAL_SUBMITFILE): $(PARTIAL_SUBMITFILES)
 	rm -f $(PARTIAL_SUBMITFILE) $(FULL_SUBMITFILE)
 	COPYFILE_DISABLE=true tar -vczf $(PARTIAL_SUBMITFILE) \
       $(PARTIAL_SUBMITFILES)
-	@echo !!! WARNING: No test cases included. Use 'make fullsubmit' to include test cases. !!!
+	@echo !!! WARNING: No test files included. Use 'make fullsubmit' to include test files. !!!
 
 # make fullsubmit.tar.gz - cleans, runs dos2unix, creates tarball
-# including test cases
+# including test files
 FULL_SUBMITFILES=$(filter-out $(TESTSOURCES), \
                    $(wildcard Makefile *.h *.hpp *.cpp test*.txt))
 $(FULL_SUBMITFILE): $(FULL_SUBMITFILES)
 	rm -f $(PARTIAL_SUBMITFILE) $(FULL_SUBMITFILE)
 	COPYFILE_DISABLE=true tar -vczf $(FULL_SUBMITFILE) $(FULL_SUBMITFILES)
-	@echo !!! Final submission prepared, test cases included... READY FOR GRADING !!!
+	@echo !!! Final submission prepared, test files included... READY FOR GRADING !!!
 
 # shortcut for make submit tarballs
 partialsubmit: identifier $(PARTIAL_SUBMITFILE)
@@ -199,7 +199,7 @@ EECS281 Advanced Makefile Help
        free submissions if the project does not build.
            $$ make partialsubmit
     B) Build 'fullsubmit.tar.gz' a tarball complete with autograder test
-       cases.
+       files.
 
            *** ALWAYS USE THIS FOR FINAL GRADING! ***