Added filename to top of .rs files

tests/tests1.rs

@@ -1,3 +1,4 @@
+// tests1.rs
 // Tests are important to ensure that your code does what you think it should do.
 // Tests can be run on this file with the following command:
 // rustc --test tests1.rs

tests/tests2.rs

@@ -1,3 +1,4 @@
+// tests2.rs
 // This test has a problem with it -- make the test compile! Make the test
 // pass! Make the test fail! Scroll down for hints :)
 

tests/tests3.rs

@@ -1,3 +1,4 @@
+// tests3.rs
 // This test isn't testing our function -- make it do that in such a way that
 // the test passes. Then write a second test that tests that we get the result
 // we expect to get when we call `is_even(5)`. Scroll down for hints!

tests/tests4.rs

@@ -1,3 +1,4 @@
+// tests4.rs
 // This test isn't testing our function -- make it do that in such a way that
 // the test passes. Then write a second test that tests that we get the result
 // we expect to get when we call `times_two` with a negative number.