Add INCLUDE-FILE: and --custom-core for file tests to specify a minimal prelude library (#5080)

The INCLUDE-FILE option is only used in the toolchain tests for now. If
specified in a file test, the given file path is added to the test's
arguments. For toolchain tests this makes the file's package available
to the test. The `--custom-core` command line flag is added to the
driver, which avoids adding the production `Core` package to the command
line. Together, these allow a test to provide their own minimal `Core`
package.

For example, this would replace `Core` with the package and prelude in
`facet_types.carbon`.
```
// INCLUDE-FILE: toolchain/testing/min_prelude/facet_types.carbon
// EXTRA-ARGS: --custom-core
```

To support this:
* //testing knows how to parse INCLUDE-FILE out of the header of a test
file.
* //testing adds the file to the virtual file system, and includes it in
the test's arguments.
* //toolchain/driver grows the --custom-core command line flag to avoid
loading the production `Core` package.

Tests that were creating their own minimal prelude to define BitAnd on
types are now pointed to
toolchain/testing/min_prelude/facet_types.carbon as the prelude. They no
longer need to `import Core` in each test as a result.

Such tests are no longer `no_prelude`, but instead have their own
prelude. So they are moved to a `min_prelude` subdirectory.

Closes #5076

testing/file_test/file_system.cpp

@@ -0,0 +1,23 @@
+// 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
+
+#include "testing/file_test/file_system.h"
+
+namespace Carbon::Testing {
+
+auto AddFile(llvm::vfs::InMemoryFileSystem& fs, llvm::StringRef path)
+    -> ErrorOr<Success> {
+  llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> file =
+      llvm::MemoryBuffer::getFile(path);
+  if (file.getError()) {
+    return ErrorBuilder() << "Getting `" << path
+                          << "`: " << file.getError().message();
+  }
+  if (!fs.addFile(path, /*ModificationTime=*/0, std::move(*file))) {
+    return ErrorBuilder() << "Duplicate file: `" << path << "`";
+  }
+  return Success();
+}
+
+}  // namespace Carbon::Testing