Ensure coverage tests run on all platforms

Whether due to platform specifics, toolchain versioning or autotools
peculiarities, the coverage tests fail to build on certain systems
without -fprofile-arcs -ftest-coverage, despite --coverage having
worked previously.

This commit rectifies the behaviour by ensuring the relevant
coverage options are enabled in AM_CFLAGS, tested on Fedora 20.

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>