Use std.Build.LazyPath over now removed std.Build.FileSource.

See corresponding ziglang change here: https://github.com/ziglang/zig/issues/16353

test/tests.zig

@@ -9,7 +9,7 @@ const mem = std.mem;
 const Allocator = std.mem.Allocator;
 const Child = std.process.Child;
 const Build = std.Build;
-const FileSource = std.Build.FileSource;
+const LazyPath = std.Build.LazyPath;
 const Reader = fs.File.Reader;
 const RunStep = std.Build.RunStep;
 const Step = Build.Step;
@@ -132,9 +132,9 @@ fn createCase(b: *Build, name: []const u8) *Step {
 const CheckNamedStep = struct {
     step: Step,
     exercise: Exercise,
-    stderr: FileSource,
+    stderr: LazyPath,
 
-    pub fn create(owner: *Build, exercise: Exercise, stderr: FileSource) *CheckNamedStep {
+    pub fn create(owner: *Build, exercise: Exercise, stderr: LazyPath) *CheckNamedStep {
         const self = owner.allocator.create(CheckNamedStep) catch @panic("OOM");
         self.* = .{
             .step = Step.init(.{
@@ -180,12 +180,12 @@ const CheckNamedStep = struct {
 const CheckStep = struct {
     step: Step,
     exercises: []const Exercise,
-    stderr: FileSource,
+    stderr: LazyPath,
 
     pub fn create(
         owner: *Build,
         exercises: []const Exercise,
-        stderr: FileSource,
+        stderr: LazyPath,
     ) *CheckStep {
         const self = owner.allocator.create(CheckStep) catch @panic("OOM");
         self.* = .{