Bug 1385172 - Replace nsEscapeHTML{,2}() with new nsAppendEscapedHTML() function. r=erahm.

The existing functions work with C strings but almost all the call sites use
Mozilla strings.

The replacement function has the following properties.

- It works with Mozilla strings, which makes it much simpler and also improves
  the call sites.

- It appends to the destination string because that's what a lot of the call
  sites need. For those that don't, we can just append to an empty string.

- It is declared outside the |extern "C"| section because there is no need for
  it to be in that section.

Note: there is no 16-bit variant of nsAppendEscapedHTML(). This is because
there are only two places that need 16-bit variants, both rarely executed,
and so converting to and from 8-bit is good enough.

The patch also adds some testing of the new function, renaming
TestEscapeURL.cpp as TestEscape.cpp in the process, because that file is now
testing other kinds of escaping.

toolkit/components/extensions/WebExtensionPolicy.cpp

@@ -251,7 +251,7 @@ WebExtensionPolicy::BackgroundPageHTML() const
 
   for (auto& script : mBackgroundScripts.Value()) {
     nsCString escaped;
-    escaped.Adopt(nsEscapeHTML(NS_ConvertUTF16toUTF8(script).get()));
+    nsAppendEscapedHTML(NS_ConvertUTF16toUTF8(script), escaped);
 
     result.AppendPrintf(kBackgroundPageHTMLScript, escaped.get());
   }