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_test_base_test.cpp

@@ -264,9 +264,9 @@ auto FileTestBaseTest::Run(
   PrintArgs(test_args, output_stream);
 
   auto filename = std::filesystem::path(test_name().str()).filename();
-  if (filename == "args.carbon") {
-    // 'args.carbon' has custom arguments, so don't do regular argument
-    // validation for it.
+  if (filename == "args.carbon" || filename == "include_file.carbon") {
+    // 'args.carbon' and 'include_file.carbon' have custom arguments, so don't
+    // do regular argument validation for them.
     return {{.success = true}};
   }