Run tests using http requests

We now start a local HTTP server on port 2900 that serves the test fixtures. This is the same port we also use in the JS test suite

fixes #532
pass array with a single nullptr instead of nullptr for compatibility
replace strftime_l with our custom formatter
use execv and pass non-inline argument for gcc
use dirname() on copies of the parameter only

glibc's dirname() modifies the parameter
Merge pull request #540 from mapbox/test-http

Run tests using HTTP requests