add another file to the integration test

tests/run-make/checksum-freshness/expected.d

@@ -1,3 +1,4 @@
-lib.d: lib.rs # checksum:blake3=5fd6328c93a3360bc580567aec10731ab3e5efc4a0f957b1bba0db0d27af3767 file_len:110
+lib.d: lib.rs foo.rs # checksum:blake3=94af75ee4ed805434484c3de51c9025278e5c3ada2315e2592052e102168a503 file_len:120 checksum:blake3=2720e17bfda4f3b2a5c96bb61b7e76ed8ebe3359b34128c0e5d8032c090a4f1a file_len:119
 
 lib.rs:
+foo.rs:

tests/run-make/checksum-freshness/foo.rs

@@ -0,0 +1,5 @@
+// This is another file, just to prove we can handle two of them
+
+pub fn subtract(a: i32, b: i32) -> i32 {
+    a - b
+}

tests/run-make/checksum-freshness/lib.rs

@@ -1,5 +1,7 @@
 // A basic library to be used in tests with no real purpose.
 
+mod foo;
+
 pub fn sum(a: i32, b: i32) -> i32 {
     a + b
 }