* Add options for parsing test files
This change allows adding parsing parameters more easily; e.g. a parameter is needed for setting the default calling convention for functions parsed as a part of the `run` test feature.
* Set default calling convention that of the host for `test run` file tests
Previously `test run` used the parser's hard-coded CallConv::Fast as the default calling convention but with this change any test being `run` will use the default calling convention of the machine running the test. `test run` will now throw an error if the calling convention of the function does not match the host's.
