Ignore ExpectationFailedError when thrown by the body closure passed to withKnownIssue

[stmontgomery]

This fixes an issue where a redundant, second known issue is recorded when try #require() fails within the body closure of withKnownIssue. For example, in the following example code:

@Test func example() {
  withKnownIssue {
    try #require(Bool(false))
  }
}

two (known) issues are recorded:

✘ Test example() recorded a known issue at KnownIssueTests.swift:641:9: Expectation failed: Bool(false)
✘ Test example() recorded a known issue at KnownIssueTests.swift:640:18: An API was misused
↳ Recorded an error of type ExpectationFailedError representing an expectation that failed and was already recorded: ...

Of these, only the first should be recorded and the second should be suppressed.

Fixes rdar://153550847

Motivation:

It should be possible to use try #require() in the body closure passed to withKnownIssue, and matching failures should only cause a single known issue to be recorded. The second issue shown above describes an infrastructural Error-conforming type which is not generally intended to be included in the results test authors see, and can cause confusion.

Modifications:

• When attempting to match an error thrown by a withKnownIssue body closure, ignore the error if it's of type `ExpectationFailedError.
• Add a new unit test validating this, which fails prior to the fix.

Checklist:

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

[briancroom]

This looks great, thanks!