add test for std.fs.Path.file_name()

lib/std/src/fs/Path.ri

@@ -85,7 +85,7 @@ public struct Path {
         if path.is_empty() {
             return none;
         }
-        
+
         mut end_index: uint := path.len - 1;
         while path[end_index] == SEPARATOR : end_index -= 1 {
             if end_index == 0 {
@@ -143,9 +143,13 @@ public struct Path {
     /// Files that end with `.`, or that start with `.` and have no other `.` in their
     /// name, are considered to have no extension.
     public func extension(path: string) -> string {
-        filename := Self.file_name(path);
+        filename := Self.base_name(path);
         index := filename.last_index_of_byte(b'.') ?? return "";
-        return if index == 0 or index + 1 >= filename.len { "" } else { filename[index..] };
+        return if index == 0 or index + 1 >= filename.len {
+            ""
+        } else {
+            filename[index..]
+        };
     }
 
     /// This function is like a series of `cd` statements executed one after another.

lib/std/tests/fs_test.ri

@@ -86,6 +86,13 @@ test "std.fs.Path.base_name()" {
     @assert(Path.base_name("foo") == "foo");
 }
 
+test "std.fs.Path.file_name()" {
+    @assert(Path.file_name("rivet/lib/std/project.json") == "project.json");
+    @assert(Path.file_name("rivet/lib/std/") == "");
+    @assert(Path.file_name("rivet/lib/std") == "std");
+    @assert(Path.file_name("filename") == "filename");
+}
+
 test "std.fs.Path.extension()" {
     @assert(Path.extension("") == "");
     @assert(Path.extension(".") == "");