Migrate explorer tests to file_test and remove lit support. (#3050)

This migrates explorer tests to file_test, using the new --autoupdate
functionality. Per discussion, the trace tests that were using "not"
output are mostly migrated to checking full output. The main exception
is tests that were including trace output from the prelude: the prelude
output is pretty long (multiple MB already) and I think it wasn't the
intent to include, only trace output from the small program.

This is the remaining use of lit support, so the supporting libraries
are also removed here.

explorer/testdata/basic_syntax/zero.carbon

@@ -0,0 +1,12 @@
+// 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
+//
+// AUTOUPDATE
+// CHECK:STDOUT: result: 0
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  return 0;
+}