Reserve the 'play' content kind in _TestDiscovery (#1090)

This defines the `'play'` test content kind in the `TestContentKind`
enum and corresponding documentation.

### Motivation:

This is intended to "reserve" this FourCC code for eventual use by the
[swift-play-experimental](https://github.com/apple/swift-play-experimental)
project, which has adopted the `_TestDiscovery` library from this
package.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Fixes #1089
Fixes rdar://147585572

Author: stmontgomery

Documentation/ABI/TestContent.md

@@ -100,9 +100,10 @@ record's kind is a 32-bit unsigned value. The following kinds are defined:
 | `0x00000000` | – | Reserved (**do not use**) |
 | `0x74657374` | `'test'` | Test or suite declaration |
 | `0x65786974` | `'exit'` | Exit test |
+| `0x706c6179` | `'play'` | [Playground](https://github.com/apple/swift-play-experimental) |
 
-<!-- When adding cases to this enumeration, be sure to also update the
-corresponding enumeration in TestContentGeneration.swift. -->
+<!-- The kind values listed in this table should be a superset of the cases in
+the `TestContentKind` enumeration. -->
 
 If a test content record's `kind` field equals `0x00000000`, the values of all
 other fields in that record are undefined.