Remove useless readdir() calls in Dir::isEmpty()

simgear::Dir::isEmpty() used to make up to 5 calls to readdir(), while 3
are enough to say whether the directory has entries other than '.' and
'..'.

Also add an automated test for this method.

simgear/misc/sg_dir.cxx

@@ -296,7 +296,10 @@ bool Dir::isEmpty() const
 
   int n = 0;
   dirent* d;
-  while( (d = readdir(dp)) !=NULL && (n < 4) ) n++;
+  while (n < 3 && (d = readdir(dp)) != nullptr) {
+    n++;
+  }
+
   closedir(dp);
 
   return (n == 2); // '.' and '..' always exist

simgear/misc/sg_dir_test.cxx

@@ -2,6 +2,7 @@
 
 #include <cstdlib>
 
+#include <simgear/io/iostreams/sgstream.hxx>
 #include <simgear/misc/sg_path.hxx>
 #include <simgear/misc/test_macros.hxx>
 #include "sg_dir.hxx"
@@ -34,11 +35,35 @@ void test_tempDir()
     d.remove();
 }
 
+void test_isEmpty()
+{
+    simgear::Dir d = simgear::Dir::tempDir("FlightGear");
+    SG_VERIFY(!d.isNull() && d.exists() && d.isEmpty());
+    SGPath f = d.file("some file");
+
+    { sg_ofstream file(f); }    // create and close the file
+    SG_VERIFY(!d.isEmpty());
+
+    f.remove();
+    SG_VERIFY(d.isEmpty());
+
+    simgear::Dir subDir{d.file("some subdir")};
+    subDir.create(0777);
+    SG_VERIFY(!d.isEmpty());
+
+    subDir.remove();
+    SG_VERIFY(d.isEmpty());
+
+    d.remove();
+    SG_VERIFY(d.isEmpty());     // eek, but that's how it is
+}
+
 int main(int argc, char **argv)
 {
     test_isNull();
     test_setRemoveOnDestroy();
     test_tempDir();
+    test_isEmpty();
 
     return EXIT_SUCCESS;
 }