test: Add support for EGL-based test, (and one EGL-based test)

For this, common.c now supports a new macro COMMON_USE_EGL which can
optionally be defined by the test before including common.c.

Some aspects of the common.c interface have changed slightly, (the
create_context call is now either create_glx_context or
create_egl_context, and the caller must explicitly call the new
common_make_current call).

This commit adds a single egl-based test, (egl-opengl-link-call),
which is similar to the existing gl-link-call test. This is basically
to ensure that the new code in common.c is functional.

We plan to follow up with egl-opengl variants for the remaining 5
existing gl tests, (and then egl-glesv2 variants for all 6 as well).