Add isFailure to ABI.EncodedIssue (#1211)

This adds `isFailure` to `ABI.EncodedIssue`

### Motivation:

Clients need to know if an issue is failing so that they know how to
handle it.

### Modifications:

This adds `isFailure` to `ABI.EncodedIssue`

### 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.

---------

Co-authored-by: Jonathan Grynspan <[email protected]>

Author: suzannaratcliff

Sources/Testing/ABI/Encoded/ABI.EncodedIssue.swift

@@ -28,6 +28,11 @@ extension ABI {
     ///
     /// - Warning: Severity is not yet part of the JSON schema.
     var _severity: Severity
+    
+    /// If the issue is a failing issue.
+    ///
+    /// - Warning: Non-failing issues are not yet part of the JSON schema.
+    var _isFailure: Bool
 
     /// Whether or not this issue is known to occur.
     var isKnown: Bool
@@ -50,6 +55,7 @@ extension ABI {
       case .warning: .warning
       case .error: .error
       }
+      _isFailure = issue.isFailure
       isKnown = issue.isKnown
       sourceLocation = issue.sourceLocation
       if let backtrace = issue.sourceContext.backtrace {