Convert test/unit based  tests to rspec based tests

1) Converted test/unit based tests to use rspec 
2) Common setup methods are now in spec_helper module
3) In few places, split up multiple assertions into separate tests (and grouped them logically)
4) No teardown invocation inside test method (as a side effect of 3rd step)

test/unit too can be configured to reuse setup/teardown method, but I think rspec's implementation is bit less invasive.