XDebug: collect branch/path coverage

Currently, PSpec only collects line coverage. XDebug also supports branch and path coverage. The percentage of all paths and branches that are executed during a text is a valuable metric about the tests.

Proposal

Collect XDebug branch/path coverage by enabling the XDEBUG_CC_BRANCH_CHECK option in xdebug_start_code_coverage.

Doing this will add a functions entry to the xdebug_get_code_coverage result.