TestFoundation: use bridging for CFStringGetLength

Rather than use CoreFoundation interfaces, use bridging more
aggressively to call the CoreFoundation methods.  This reduces the
dependency on CoreFoundation for the test suite.

Author: compnerd

Foundation/String.swift

@@ -12,6 +12,10 @@
 
 import CoreFoundation
 
+extension String {
+  public static let transformStripCombiningMarks: CFString = kCFStringTransformStripCombiningMarks
+}
+
 extension String : _ObjectiveCBridgeable {
 
     public typealias _ObjectType = NSString

TestFoundation/TestNSString.swift

@@ -606,7 +606,7 @@ class TestNSString: LoopbackServerTest {
         buf.reserveCapacity(reservedLength)
         var usedLen : CFIndex = 0
         let _ = buf.withUnsafeMutableBufferPointer { p in
-            CFStringGetBytes(cfString, CFRangeMake(0, CFStringGetLength(cfString)), CFStringEncoding(kCFStringEncodingUTF16), 0, false, p.baseAddress, reservedLength, &usedLen)
+            CFStringGetBytes(cfString, CFRangeMake(0, (cfString as! NSString).length), CFStringEncoding(kCFStringEncodingUTF16), 0, false, p.baseAddress, reservedLength, &usedLen)
         }
 
         // Make a new string out of it
@@ -1299,11 +1299,11 @@ class TestNSString: LoopbackServerTest {
     }
 
     func test_substringFromCFString() {
-        let cfString = kCFStringTransformStripCombiningMarks!
-        let range = NSRange(location: 0, length: CFStringGetLength(cfString))
+        let cfString = String.transformStripCombiningMarks
+        let range = NSRange(location: 0, length: unsafeBitCast(cfString, to: NSString.self).length)
         let substring = unsafeBitCast(cfString, to: NSString.self).substring(with: range)
 
-        XCTAssertEqual(CFStringGetLength(cfString), substring.utf16.count)
+        XCTAssertEqual(unsafeBitCast(cfString, to: NSString.self).length, substring.utf16.count)
     }
 
     func test_createCopy() {