Rust: add nested test file example

rust/ql/test/extractor-tests/File/File.expected

@@ -2,3 +2,5 @@
 | a_file.rs:0:0:0:0 | a_file.rs | fromSource: yes |
 | another_file.rs:0:0:0:0 | another_file.rs | fromSource: yes |
 | lib.rs:0:0:0:0 | lib.rs | fromSource: yes |
+| nested.rs:0:0:0:0 | nested.rs | fromSource: yes |
+| nested/file.rs:0:0:0:0 | nested/file.rs | fromSource: yes |

rust/ql/test/extractor-tests/File/nested.rs

@@ -0,0 +1 @@
+mod file;