Remove imageSupportRequired parameter to runTestHarness (#1077)

* Tests requiring image support use runTestHarnessWithCheck

Removing special case for images in runTestHarness.

Fixes #710

* Remove imageSupportRequired argument

Tests which require image support now specify this while
calling runTestHarnessWithCheck.

Fixes #710

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