add tests for gaphml with strict xml checks

[GREENRAT-K405]

In this PR, I have added tests for graphml file in tests/test_graph.py:

• test_validate_corrupted_xml: test for files with unclosed tags
• test_validate_empty_file: test for empty file
• test_validate_missing_root_element: incase there is XML file but it is not a graphml file
• test_validate_missing_node_id: test for node id attributes
• test_validate_missing_edgedefault
• test_validate_broken_edges: test for cases like edge exists but the node is missing
• test_validate_node_missing_filename: for test case like node exists but it isn't linked to any concore program
• test_validate_valid_graph: test case for a perfectly minimal graphml file

the major changes in the validate.py:

• added xml.etree.ElementTree to detect and report broken XML syntax immediately.
• Added mandatory checks for:
  -- The edgedefault attribute on the element, it has to be directed or undirected.
  --The node "id" attribute on every , every node must have node id.

I have checked these test by building some dummy sample graphs:

1. empty graph shows warning-> no nodes, no edges

2. graph with just node (no program attached with it) but no edge

3. 

4. 

5. 

[GREENRAT-K405]

@pradeeban If anymore tests are required please do let me know.