Start including the prelude for testing. (#3861)

- Adds an empty prelude.carbon file
- Imports that file in any non-Core package file
  - Adds --disable-prelude-import to avoid that
- Adds --exclude-dump-file-prefix to be able to hide files from dumping
- Used to hide core files (we can't do this by package name due to lex
dumps, for example)
- Restructures some tests to not rely on `i32`, particularly `alias`
tests (which rely on a name ref) and tests with no prelude.

I'm adding the framework for switching i32 to calling Int32 in the
prelude, but I'm running into a separate error actually switching over.
So that *mostly* works, but isn't quite ready for prime time. However,
maybe the current state of this PR is still useful to review since it
does a lot of the infrastructure work and adds the %Core everywhere?

toolchain/check/node_stack.h

@@ -502,7 +502,9 @@ class NodeStack {
   auto RequireIdKind(Parse::NodeKind parse_kind, Id::Kind id_kind) const
       -> void {
     CARBON_CHECK(NodeKindToIdKind(parse_kind) == id_kind)
-        << "Unexpected Id::Kind mapping for " << parse_kind;
+        << "Unexpected Id::Kind mapping for " << parse_kind << ": expected "
+        << static_cast<int>(id_kind) << ", found "
+        << static_cast<int>(NodeKindToIdKind(parse_kind));
   }
 
   // Require an entry to have the given Parse::NodeKind.