testsuite: If gsk_renderer_realize() fails, say why
Showing the error message is usually more useful for debugging than just saying that something failed.
I'm seeing a test failure in this location on Debian riscv64, which turns out to be that when using GALLIUM_DRIVER=softpipe
to work around LLVM JIT and therefore llvmpipe being known-broken on this architecture, initializing the EGL display fails.
The test failure is probably more of a Mesa problem than a GTK problem, but the ability to diagnose it is useful.
Edited by Simon McVittie