TestFoundation: don't use hard-coded TestURL test directory on Android

Use NSTemporaryDirectory() instead and reset the working directory
when done.

Author: finagolfin

TestFoundation/TestURL.swift

@@ -286,6 +286,7 @@ class TestURL : XCTestCase {
     static var gBaseCurrentWorkingDirectoryPath : String {
         return FileManager.default.currentDirectoryPath
     }
+    static var gSavedPath = ""
     static var gRelativeOffsetFromBaseCurrentWorkingDirectory: UInt = 0
     static let gFileExistsName = "TestCFURL_file_exists\(ProcessInfo.processInfo.globallyUniqueString)"
     static let gFileDoesNotExistName = "TestCFURL_file_does_not_exist"
@@ -339,9 +340,8 @@ class TestURL : XCTestCase {
             }
         }
 
-        #if os(Android)
-        FileManager.default.changeCurrentDirectoryPath("/data/local/tmp")
-        #endif
+        TestURL.gSavedPath = FileManager.default.currentDirectoryPath
+        FileManager.default.changeCurrentDirectoryPath(NSTemporaryDirectory())
 
         let cwd = FileManager.default.currentDirectoryPath
         let cwdURL = URL(fileURLWithPath: cwd, isDirectory: true)
@@ -358,6 +358,7 @@ class TestURL : XCTestCase {
             let error = strerror(errno)!
             XCTFail("Failed to set up test paths: \(String(cString: error))")
         }
+        defer { FileManager.default.changeCurrentDirectoryPath(TestURL.gSavedPath) }
 
         // test with file that exists
         var path = TestURL.gFileExistsPath
@@ -403,6 +404,7 @@ class TestURL : XCTestCase {
             let error = strerror(errno)!
             XCTFail("Failed to set up test paths: \(String(cString: error))")
         }
+        defer { FileManager.default.changeCurrentDirectoryPath(TestURL.gSavedPath) }
 
         // test with file that exists
         var path = TestURL.gFileExistsPath