Add a new option to allow partial test output for non-Ok() messages.

Adds a new `WithAllowPartialOutput()` method to `TextOutputOptions` to
allow `WriteToString()` to write partial output when called with a
non-`Ok()` view.
diff --git a/testdata/requires.emb b/testdata/requires.emb
index b37f61a..3317a98 100644
--- a/testdata/requires.emb
+++ b/testdata/requires.emb
@@ -85,3 +85,10 @@
     if b_exists:
       2 [+1]  Flag b_true
         [requires: this]
+
+
+struct RequiresInArrayElements:
+  struct Element:
+    0 [+1]  UInt:8  x  [requires: 0 <= this <= 10]
+
+  0 [+4]  Element[]  xs