Fix String.deletingPathExtension Inconsistency

The following code behaves differently on macOS and Linux:

```swift
NSString(string: "..").deletingPathExtension
```

On Linux, the resulting value is `.` while on macOS, it's `..`.

I think the latter make far more sense since `..` mean parent directly,
treating the last dot as "extension" is a bit of a stretch. Plus, this
logic existed on macOS longer (therefore has more existing users). So it
make sense to align Linux Foundation's behavior to that of macOS.

Foundation/NSPathUtilities.swift

@@ -68,6 +68,8 @@ internal extension String {
             } else if char == "." {
                 if lastCompStartPos == prevPos {
                     return nil
+                } else if case let prevPrevPos = index(before: prevPos), prevPos == index(before: endIndex) && prevPrevPos == lastCompStartPos && self[prevPrevPos] == "." {
+                    return nil
                 } else {
                     return curPos
                 }
@@ -262,7 +264,7 @@ public extension NSString {
         if fixedSelf.length <= 1 {
             return fixedSelf
         }
-        if let extensionPos = (fixedSelf._startOfPathExtension) {
+        if let extensionPos = fixedSelf._startOfPathExtension {
             return String(fixedSelf.prefix(upTo: fixedSelf.index(before: extensionPos)))
         } else {
             return fixedSelf

TestFoundation/TestNSString.swift

@@ -1078,6 +1078,7 @@ class TestNSString : XCTestCase {
             NSString(string: "scratch..tiff") : "scratch.",
             NSString(string: ".tiff") : ".tiff",
             NSString(string: "/") : "/",
+            NSString(string: "..") : "..",
         ]
         for (fileName, expectedResult) in values {
             let result = fileName.deletingPathExtension