repl: execute REPL tests ~1.5x to 2x faster

vlib/compiler/tests/repl/chained_fields/bd.repl

@@ -0,0 +1,17 @@
+struct A { mut: v int }  struct B { a A }  struct C { mut: b B } struct D { mut: c C }
+mut b := B{} b = B{A{2}}
+b.a.v = 1   // Error (field a immutable)
+b.a = A{}   // Error (field a immutable)
+===output===
+cannot modify immutable field `a` (type `B`)
+declare the field with `mut:`
+struct B {
+mut:
+	a A
+}
+cannot modify immutable field `a` (type `B`)
+declare the field with `mut:`
+struct B {
+mut:
+	a A
+}