[stdlibunittest] WIP uniformity/de-boilerplating

The way we pass and compose source locations, messages, etc. needs to be
brought under control before too many more tests get written.  This is
the first step.

Swift SVN r29928

validation-test/stdlib/UnicodeUTFEncoders.swift

@@ -125,10 +125,11 @@ class CodecTest<Codec : TestableUnicodeCodec> {
 
     encodeIndex = encodeBuffer.startIndex
     Codec.encode(scalar, output: encodeOutput)
-    expectEqual(nsEncoded, encodeBuffer[0..<encodeIndex])  {
+    expectEqual(
+      nsEncoded, encodeBuffer[0..<encodeIndex],
       "Decoding failed: \(asHex(nsEncoded)) => " +
         "\(asHex(scalar.value)) => \(asHex(self.encodeBuffer[0]))"
-    }
+    )
   }
 
   func run(minScalarOrd: Int, _ maxScalarOrd: Int) {