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/run_test.cpp

@@ -11,6 +11,7 @@
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/PrettyStackTrace.h"
+#include "testing/file_test/file_system.h"
 #include "testing/file_test/file_test_base.h"
 #include "testing/file_test/test_file.h"
 
@@ -93,6 +94,9 @@ auto RunTestFile(const FileTestBase& test_base, bool dump_output,
     test_file.test_args = test_base.GetDefaultArgs();
     test_file.test_args.append(test_file.extra_args);
   }
+  for (const auto& include_file : test_file.include_files) {
+    test_file.test_args.push_back(include_file);
+  }
   CARBON_RETURN_IF_ERROR(DoArgReplacements(test_file.test_args,
                                            test_base.GetArgReplacements(),
                                            test_file.file_splits));
@@ -123,6 +127,11 @@ auto RunTestFile(const FileTestBase& test_base, bool dump_output,
       return ErrorBuilder() << "File is repeated: " << split.filename;
     }
   }
+
+  for (llvm::StringRef file_path : test_file.include_files) {
+    CARBON_RETURN_IF_ERROR(AddFile(*fs, file_path));
+  }
+
   // Convert the arguments to StringRef and const char* to match the
   // expectations of PrettyStackTraceProgram and Run.
   llvm::SmallVector<llvm::StringRef> test_args_ref;