-Running the FileSystem tests no longer leaves test directories or files in the running directory. fixes #1412

-Modernized the tests for GetFIleSystemEntries and GetFiles, splitting each test case into a standalone method runnable through xunit. Similar versions of these tests now inherit from each other rather than just being copy-pasted like they were previously. Moved most test cases from using hardcoded file names to instead use GetTestFileName() in FileSystemTest.cs. Tested on Linux and Windows.
-Changed the names of some files and classes to better follow the naming conventions of the other FileSystem tests.

Author: Ian Hays

src/System.IO.FileSystem/tests/Directory/Delete_str_bool.cs

@@ -10,7 +10,7 @@
 using System.Runtime.CompilerServices;
 using Xunit;
 
-public class Directory_Co5663Delete_str_bool
+public class Directory_Delete_str_bool
 {
     public static String s_strDtTmVer = "2000/04/26 20:33";
     public static String s_strClassMethod = "Directory.Delete(String,Boolean)";

src/System.IO.FileSystem/tests/Directory/GetCreationTime_str.cs

@@ -9,7 +9,7 @@
 using System.Collections.Generic;
 using Xunit;
 
-public class Directory_GetCreationTime
+public class Directory_GetCreationTime_str
 {
     public static String s_strDtTmVer = "2009/02/18";
     public static String s_strClassMethod = "Directory.GetCreationTime()";

src/System.IO.FileSystem/tests/Directory/GetDirectories_str_str.cs

@@ -10,7 +10,7 @@
 using System.Runtime.CompilerServices;
 using Xunit;
 
-public class Directory_Co5674GetDirectories_Str_Str
+public class Directory_GetDirectories_Str_Str
 {
     public static String s_strDtTmVer = "2000/04/27 17:00";
     public static String s_strClassMethod = "Directory.GetDirectories()";