Change the default value of testcase.overwrite to 'force'

Python raised ValueError while reading a sample input because int() cannot parse long integers with 'L' like '1L'.

Values are printed as-is in PythonLanguage#renderParamValue(). However, Java is selected first in topcoder arena. Therefore, long integers are printed with 'L' in JavaLanguage#renderParamValue(). Though I changed language to Python in arena, *.sample file was not updated.

To fix this issue, the default value of testcase.overwrite should be 'force'.