Added tests for djblets.log.views for testing the parse_timestamp

function.
The included tests try the following cases with provided timestamp

format string:

- Empty String of '' being passed

- A string containing something other than a date ('jibberish')

- An invalid date where the month is out of bounds 

('1990-55-09 22:11:30')

- A valid date input string ('1990-01-01 20:20:20')
Based most of this behavior off how parse_timestamp behaved in the

interpreter with various inputs. To be more accurate with testing

would require the default implementation for time and datetime,

which mightbe something to be considered with future changes to

python versions however but isn't required for 

the scope of this project.

I ran  ./tests/runtests djblets.log.tests and

./tests/runtests djblets.log and both pass along with 

all other test cases.