Automate the addition of RUN and simplify RUN lines (#2292)

This was an offshoot of the discussion about how much boilerplate we could remove. lit requires RUN lines be there, everything else is optional.

explorer/testdata/assert/fail_assert.carbon

@@ -0,0 +1,20 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// This won't auto-update because it uses a regex for the prelude line number,
+// so that it doesn't need to be updated on every prelude change.
+//
+// This can't be autoupdated because the error line comes form a different file.
+// NOAUTOUPDATE
+// RUN: %{not} %{explorer-run}
+// RUN: %{not} %{explorer-run-trace}
+// CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/data/prelude.carbon:{{.*}}: "HALLO WELT"
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  Assert(true, "HALLO WELT");
+  Assert(false, "HALLO WELT");
+  return 0;
+}