Expose Issue severity and isFailure as API

[suzannaratcliff]

Expose Issue Severity and isFailure as API

Motivation:

To allow users to create issues on their tests without causing the tests to fail, it would be helpful to support creating issues with a warning severity.

Test Warnings:
Severity:

  public enum Severity: Sendable {
    /// The severity level for an issue which should be noted but is not
    /// necessarily an error.
    ///
    /// An issue with warning severity does not cause the test it's associated
    /// with to be marked as a failure, but is noted in the results.
    case warning

    /// The severity level for an issue which represents an error in a test.
    ///
    /// An issue with error severity causes the test it's associated with to be
    /// marked as a failure.
    case error
  }

This will allow users to create issues but they will not have to fail the test

Example Usage:

Issue.record("My comment", severity: .warning)

Later on users can inspect the severity of their issue with the severity property.

/// The severity of the issue.
public var severity: Severity

  /// Whether or not this issue should cause the test it's associated with to be
  /// considered a failure.
  ///
  /// The value of this property is `true` for issues which have a severity level of
  /// ``Issue/Severity/error`` or greater and are not known issues via
  /// ``withKnownIssue(_:isIntermittent:sourceLocation:_:when:matching:)``.
  /// Otherwise, the value of this property is `false.`
  ///
  /// Use this property to determine if an issue should be considered a failure, instead of
  /// directly comparing the value of the ``severity`` property.
public var isFailure: Bool

This API will be useful for teams who would like to inspect if their issue is a failing issue
https://developer.apple.com/documentation/xctest/xctestcase/record(_:)

Modifications:

• Add isFailure variable
• Remove experimental spi from Severity

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

I don't think this member is necessary; whether or not a warning is a failure might depend on external configuration (some equivalent to -Werror perhaps.)