Replace os.sep with '/' in _check_path()

Use a hard-coded unix separator ('/') so that an
exception is also raised for paths starting with '/'
when executing on Windows systems.

Update test_check_path to explicitly test invalid paths
starting with Windows style separator.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

tests/test_repository_tool.py

@@ -1677,10 +1677,6 @@ def test_add_paths(self):
     # paths, which it previously did.
     self.targets_object.add_paths(['non-existent'], 'tuf')
 
-    # add_paths() should not raise an exception for paths that
-    # are not located in the repository's targets directory
-    repository_directory = os.path.join('repository_data', 'repository')
-    self.targets_object.add_paths([repository_directory], 'tuf')
 
 
 
@@ -1724,11 +1720,16 @@ def test_check_path(self):
     self.assertRaises(securesystemslib.exceptions.FormatError,
         self.targets_object._check_path, 3)
 
-    # Test invalid pathname - starting with os separator
+    # Test invalid pathname
+    # Starting with os separator
     self.assertRaises(tuf.exceptions.InvalidNameError,
         self.targets_object._check_path, '/file1.txt')
 
-    # Test invalid pathname - using '\' as os separator
+    # Starting with Windows-style separator
+    self.assertRaises(tuf.exceptions.InvalidNameError,
+        self.targets_object._check_path, '\\file1.txt')
+
+    # Using Windows-style separator ('\')
     self.assertRaises(tuf.exceptions.InvalidNameError,
         self.targets_object._check_path, 'subdir\\non-existent')
 

tuf/repository_tool.py

@@ -2784,7 +2784,7 @@ def _check_path(self, pathname):
       raise tuf.exceptions.InvalidNameError('Path ' + repr(pathname)
           + ' does not use the forward slash (/) as directory separator.')
 
-    if pathname.startswith(os.sep):
+    if pathname.startswith('/'):
       raise tuf.exceptions.InvalidNameError('Path ' + repr(pathname)
           + ' starts with a directory separator. All paths should be relative'
           '  to targets directory.')