Re-enable some tests

[slozier]

Notes:

• Disabling test_linecache since moving it to test_linecache_stdlib makes the failure disappear
• Disabling test_regrtest since there are lots of failures and it's not super interesting

Related issues:

• PEP 526: Syntax for variable annotations #106
• subprocess.Popen with stdout and stderr not producing any output #1135
• __loader__ not in globals #1245
• generator does not free resources after iteration #1246
• Some test cases try to run ipy with -I and fail in release mode #1440
• PEP 487 – Simpler customisation of class creation #1448
• backslashreplace error handlers should work for decoding and translating #1452
• PEP 468 – Preserving the order of **kwargs in a function #1460
• dict should be insertion-order preserving #1556
• Exception context not set in certain cases #1557