Allow args, includes, and splits in file_test includes (#5610)

This builds a mechanism for the toolchain to construct more complex
min_prelude files, which in turn should allow the toolchain to stop
special-casing the min_prelude directory. For example, instead of:

```
      args.insert(args.end(), {"--custom-core",
                               "--exclude-dump-file-prefix=include_files/"});
```

This should allow (in a `min_prelude` file):

```
// EXTRA-ARGS: --custom-core --exclude-dump-file-prefix=include_files/
```

Then when that min_prelude is included, it'd be used.

But also, this should allow sharing between min_prelude files with use
of `INCLUDE-FILE`, which as we make progressively more complex
min_preludes might become useful.

testing/file_test/testdata/include_files/split.carbon

@@ -0,0 +1,11 @@
+// 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
+
+// --- a.carbon
+
+a
+
+// --- b.carbon
+
+b