Store test content in a custom metadata section.

See also: swiftlang/swift#76698

Resolves #735.

Author: grynspan

Documentation/ABI/TestContent.md

@@ -21,14 +21,8 @@ the testing library are stored in dedicated platform-specific sections:
 
 | Platform | Binary Format | Section Name |
 |-|:-:|-|
-| macOS | Mach-O | `__DATA_CONST,__swift5_tests` |
-| iOS | Mach-O | `__DATA_CONST,__swift5_tests` |
-| watchOS | Mach-O | `__DATA_CONST,__swift5_tests` |
-| tvOS | Mach-O | `__DATA_CONST,__swift5_tests` |
-| visionOS | Mach-O | `__DATA_CONST,__swift5_tests` | 
-| Linux | ELF | `PT_NOTE`[^1] |
-| FreeBSD | ELF | `PT_NOTE`[^1] |
-| Android | ELF | `PT_NOTE`[^1] |
+| macOS, iOS, watchOS, tvOS, visionOS | Mach-O | `__DATA_CONST,__swift5_tests` |
+| Linux, FreeBSD, Android | ELF | `PT_NOTE`[^1] |
 | WASI | Statically Linked | `swift5_tests` |
 | Windows | PE/COFF | `.sw5test` |
 
@@ -66,6 +60,11 @@ struct SWTTestContent {
 };
 ```
 
+This structure may grow in the future as needed. Check the `n_descsz` field of
+the record header to determine if there are additional fields present. Do not
+assume that the size of this structure will remain fixed over time or that all
+discovered test content records are the same size.
+
 #### The accessor field
 
 The function `accessor` is a C function whose signature in Swift can be restated