[regexp] Fix a test case for RE.proto[@@search]

The test case did not test anything in its original form. Fix it and add
documentation.

BUG=v8:5339

Review-Url: https://codereview.chromium.org/2481733002
Cr-Commit-Position: refs/heads/master@{#40794}

test/mjsunit/regexp.js

@@ -741,9 +741,10 @@ assertThrows(() => "abc".replace(/./, ""));
 RegExp.prototype.exec = RegExpPrototypeExec;
 
 // Test the code path in RE.proto[@@search] when previousLastIndex is a receiver
-// but can't be converted to a primitive. This exposed a crash in the
-// C++ implementation of @@search.
+// but can't be converted to a primitive. This exposed a crash in an older
+// C++ implementation of @@search which a) still relied on Object::Equals,
+// and b) incorrectly returned isolate->pending_exception() on error.
 
 var re = /./;
 re.lastIndex = { [Symbol.toPrimitive]: 42 };
-() => "abc".search(re);
+try { "abc".search(re); } catch (_) {}  // Ensure we don't crash.