EmptyDirectoryConstraint doesn't need to enumerate entire directory contents

[stevenaw]

EmptyDirectoryConstraint currently tries to read in and instantiate all content in a directory in order to determine if the directory is empty or not empty.

https://github.com/nunit/nunit/blob/master/src/NUnitFramework/framework/Constraints/EmptyDirectoryConstraint.cs#L55-L57

An alternative could be to use one call to EnumerateFileSystemInfos().Any() to detect existence of either files or subdirectories without instantiating a whole array of them. Unfortunately, this API isn't available in .NET 35, so some sort of fallback would be required to go this route. A fallback could be either the currently implementation, or using GetFileSystemInfos().Any(). Example:

- files = dirInfo.GetFiles().Length;
- subdirs = dirInfo.GetDirectories().Length;
- bool hasSucceeded = files == 0 && subdirs == 0;

+ bool hasSucceeded;
+ #if !NET35
+           hasSucceeded = !dirInfo.EnumerateFileSystemInfos().Any();
+ #else
+           hasSucceeded = !dirInfo.GetFileSystemInfos().Any();
+ #endif

I've done a quick verification via BenchmarkDotNet, and it seems promising (N = number of files, N/2 = number of subdirectories)

|           Method |    N |        Mean |        Error |    StdDev |    Gen 0 |   Gen 1 | Gen 2 | Allocated |
|----------------- |----- |------------:|-------------:|----------:|---------:|--------:|------:|----------:|
| IsEmpty_Original |    0 |    44.18 us |    11.016 us |  0.604 us |   0.1221 |       - |     - |     592 B |
|      IsEmpty_New |    0 |    21.05 us |     2.992 us |  0.164 us |   0.0610 |       - |     - |     296 B |
| IsEmpty_Original |  100 |   115.95 us |     3.041 us |  0.167 us |  11.9629 |  0.1221 |     - |   50216 B |
|      IsEmpty_New |  100 |    61.05 us |   153.207 us |  8.398 us |   0.1221 |       - |     - |     600 B |
| IsEmpty_Original | 1024 | 1,447.30 us | 1,445.389 us | 79.227 us | 105.4688 | 33.2031 |     - |  508424 B |
|      IsEmpty_New | 1024 |    79.75 us |    31.172 us |  1.709 us |   0.1221 |       - |     - |     600 B |