Change default flags for min_prelude tests (#5471)

I think this would probably have prevented the missed include in #5469
-- it would've just failed completely with a "missing prelude"
diagnostic.

Also note this excludes the included IR from output, because it's
probably low-value to print.

toolchain/check/testdata/function/declaration/min_prelude/fail_import_incomplete_return.carbon

@@ -3,7 +3,6 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // INCLUDE-FILE: toolchain/testing/min_prelude/destroy.carbon
-// EXTRA-ARGS: --custom-core
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
@@ -276,9 +275,3 @@ fn CallFAndGIncomplete() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @ReturnDUsed [from "fail_incomplete_return.carbon"];
 // CHECK:STDOUT:
-// CHECK:STDOUT: --- include_files/destroy.carbon
-// CHECK:STDOUT:
-// CHECK:STDOUT: file {
-// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {}
-// CHECK:STDOUT: }
-// CHECK:STDOUT: