refactor: extract expander helpers and Test stdio/file setters

- Add globWithAffixes in runner/expander.go that wraps filepath.Glob
    with an empty-match check and returns the computed prefix/suffix
    from splitGlob, collapsing three near-identical lookup blocks in
    expandGlobPattern into single calls.
  - Extract per-case Test construction from buildTests into a new
    buildTest helper so the loop body is a single call and the read /
    assemble / arg-template logic lives in one place.
  - Add Test.SetInputFile, Test.SetStdin, Test.SetOutputFile and
    Test.SetStdout methods on dsl.Test to encapsulate the stdin-vs-
    InFiles and stdout-vs-OutFiles wiring that buildTest previously
    did inline.
  - Adopt the `for range N` loop form in the determinism check in
    runner/compiler_test.go.
  - Switch the MSVC release test to expect /std:c17 since MSVC does
    not ship a c11 mode (worth surfacing a warning about this later).

dsl/ast.go

@@ -76,3 +76,21 @@ type Test struct {
 	Stderr   Matcher
 	OutFiles map[string]string
 }
+
+func (t *Test) SetInputFile(inputName string, inputContent []byte) {
+	t.InFiles[inputName] = string(inputContent)
+}
+
+func (t *Test) SetStdin(inputContent []byte) {
+	s := string(inputContent)
+	t.Stdin = &s
+}
+
+func (t *Test) SetOutputFile(outputName string, outputContent []byte) {
+	t.OutFiles[outputName] = string(outputContent)
+	t.Stdout = NoMatcher{}
+}
+
+func (t *Test) SetStdout(outputContent []byte) {
+	t.Stdout = ExactMatcher{Value: string(outputContent)}
+}