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/testdata/function/declaration/fail_modifiers.carbon

@@ -98,6 +98,7 @@ extern private fn ExternOrderAndConflict() {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
 // CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .Core = %Core
 // CHECK:STDOUT:     .WrongOrder = %WrongOrder
 // CHECK:STDOUT:     .DuplicateVirtual = %DuplicateVirtual
 // CHECK:STDOUT:     .TwoAccess = %TwoAccess
@@ -106,6 +107,7 @@ extern private fn ExternOrderAndConflict() {}
 // CHECK:STDOUT:     .ModifiersConflict2 = %ModifiersConflict2
 // CHECK:STDOUT:     .ExternOrderAndConflict = %ExternOrderAndConflict
 // CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core: <namespace> = namespace [template] {}
 // CHECK:STDOUT:   %WrongOrder: <function> = fn_decl @WrongOrder [template] {}
 // CHECK:STDOUT:   %DuplicateVirtual: <function> = fn_decl @DuplicateVirtual [template] {}
 // CHECK:STDOUT:   %TwoAccess: <function> = fn_decl @TwoAccess [template] {}