Add StringOps and StringBuilder to stdlib tests

Author: odersky

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -398,8 +398,9 @@ class CheckCaptures extends Recheck, SymTransformer:
       if meth == defn.Caps_unsafeAssumePure then
         val arg :: Nil = tree.args: @unchecked
         val argType0 = recheck(arg, pt.capturing(CaptureSet.universal))
-        val argType = if argType0.captureSet.isAlwaysEmpty then argType0
-        else argType0.widen.stripCapturing
+        val argType =
+          if argType0.captureSet.isAlwaysEmpty then argType0
+          else argType0.widen.stripCapturing
         capt.println(i"rechecking $arg with ${pt.capturing(CaptureSet.universal)}: $argType")
         super.recheckFinish(argType, tree, pt)
       else if meth == defn.Caps_unsafeBox then

tests/pos/stdlib/StringOps.scala

@@ -0,0 +1 @@
+collection/StringOps.scala