Ensure is_extension_available is used where possible (#722)

(Patch2)
A number of tests have got their own code for checking the presence of
extensions. This change replaces that code with is_extension_available
function.

Contributes to #627

Signed-off-by: Ellen Norris-Thompson <ellen.norris-thompson@arm.com>

Change-Id: I17e007e5ad009e522c5006c42537bf1170550a6f
10 files changed