Xcode 12.5 support

[chenxiao0228]

Bluepill crashes on Xcode 12.5 either with

'NSInvalidArgumentException', reason: '-[__NSCFString identifierString]: unrecognized selector sent to instance 0x7fdb5d606b30'

or with

error: -[BluepillTests testRunUITest] : Selector _IDE_initiateControlSessionForTestProcessID:protocolVersion: is not part of the remote interface "XCTestManager_DaemonConnectionInterface" (NSGenericException)

[YangSun33]

After some investigation, here are some issues I found caused by the changes in 12.5 SDK:

1. In XCTestConfiguration, testsToRun & testsToSkip change from NSSet to XCTTestIdentifierSet, and the elements inside change from NSString to XCTTestIdentifier.
   Issue:
   The test app will crash as it tries to access [element identifierString], which is available for XCTTestIdentifier but not for NSString.
   Solution:
   a) Change from the infra level for BPConfiguration: make testCasesToSkip, testCasesToRun & allTestCases to be arrays of XCTTestIdentifier rather than NSString.
   b) Workaround by adding a category for NSString, returning self for selector 'identifierString'.

2. The functions in protocol XCTestManager_DaemonConnectionInterface have been deprecated.
   Issue:
   The test app could be launched but no tests would be run due to connection failed.
   Facts:
   We call function _IDE_initiateControlSessionForTestProcessID:protocolVersion: in [BPTestDaemonConnection connect], and it has been refactored into protocol XCTMessagingRole_ControlSessionInitiation.
   We call function _IDE_initiateSessionWithIdentifier:forClient:atPath:protocolVersion: in [BPTestBundleConnection connect], and it has been refactored into protocol XCTMessagingRole_RunnerSessionInitiation.
   We need to change the logics in BPTestDaemonConnection & BPTestBundleConnection to get the correct object in DTXConnection so that the functions can be called.
   Solution:
   For now, I'm not sure what to do for this issue.