Meson tests cleanups

• Move tests to suites so that it will be possible to run tests with something like

      meson test --suite [core|cogl|clutter] [single-test-name]

• Add stacking tests to the list that are missing
• Split stacking and cogl conform tests so that each one is ran as single test
• Cogl unit tests now runs again

So, meson --list reports these:

mutter:cogl+cogl/conform / pipeline-user-matrix
mutter:cogl+cogl/conform / blend-strings
mutter:cogl+cogl/conform / blend
mutter:cogl+cogl/conform / premult
mutter:cogl+cogl/conform / path
mutter:cogl+cogl/conform / path-clip
mutter:cogl+cogl/conform / depth
mutter:cogl+cogl/conform / color-mask
mutter:cogl+cogl/conform / backface-culling
mutter:cogl+cogl/conform / layer-remove
mutter:cogl+cogl/conform / sparse-pipeline
mutter:cogl+cogl/conform / npot-texture
mutter:cogl+cogl/conform / sub-texture
mutter:cogl+cogl/conform / pixel-buffer-map
mutter:cogl+cogl/conform / pixel-buffer-set-data
mutter:cogl+cogl/conform / pixel-buffer-sub-region
mutter:cogl+cogl/conform / texture-3d
mutter:cogl+cogl/conform / wrap-modes
mutter:cogl+cogl/conform / texture-get-set-data
mutter:cogl+cogl/conform / atlas-migration
mutter:cogl+cogl/conform / read-texture-formats
mutter:cogl+cogl/conform / write-texture-formats
mutter:cogl+cogl/conform / alpha-textures
mutter:cogl+cogl/conform / wrap-rectangle-textures
mutter:cogl+cogl/conform / primitive
mutter:cogl+cogl/conform / just-vertex-shader
mutter:cogl+cogl/conform / pipeline-uniforms
mutter:cogl+cogl/conform / snippets
mutter:cogl+cogl/conform / custom-attributes
mutter:cogl+cogl/conform / offscreen
mutter:cogl+cogl/conform / framebuffer-get-bits
mutter:cogl+cogl/conform / point-size
mutter:cogl+cogl/conform / point-size-attribute
mutter:cogl+cogl/conform / point-size-attribute-snippet
mutter:cogl+cogl/conform / point-sprite
mutter:cogl+cogl/conform / point-sprite-orientation
mutter:cogl+cogl/conform / point-sprite-glsl
mutter:cogl+cogl/conform / alpha
mutter:cogl+cogl/conform / map-buffer-range
mutter:cogl+cogl/conform / primitive-and-journal
mutter:cogl+cogl/conform / copy-replace-texture
mutter:cogl+cogl/conform / pipeline-cache-unrefs-texture
mutter:cogl+cogl/conform / pipeline-shader-state
mutter:cogl+cogl/conform / gles2-context
mutter:cogl+cogl/conform / gles2-context-fbo
mutter:cogl+cogl/conform / gles2-context-copy-tex-image
mutter:cogl+cogl/conform / euler-quaternion
mutter:cogl+cogl/conform / color-hsl
mutter:cogl+cogl/conform / fence
mutter:cogl+cogl/conform / texture-no-allocate
mutter:cogl+cogl/conform / texture-rg
mutter:cogl+cogl/unit / check-bitmask-api
mutter:cogl+cogl/unit / check-blend-constant-ancestry
mutter:cogl+cogl/unit / check-gl-blend-enable
mutter:cogl+cogl/unit / check-mesa-driver-package-parser
mutter:cogl+cogl/unit / check-pipeline-pruning
mutter:cogl+cogl/unit / check-point-size-shader
mutter:cogl+cogl/unit / check-uniform-ancestry
mutter:clutter+clutter/conform / actor-anchors
mutter:clutter+clutter/conform / actor-destroy
mutter:clutter+clutter/conform / actor-graph
mutter:clutter+clutter/conform / actor-invariants
mutter:clutter+clutter/conform / actor-iter
mutter:clutter+clutter/conform / actor-layout
mutter:clutter+clutter/conform / actor-meta
mutter:clutter+clutter/conform / actor-offscreen-redirect
mutter:clutter+clutter/conform / actor-paint-opacity
mutter:clutter+clutter/conform / actor-pick
mutter:clutter+clutter/conform / actor-shader-effect
mutter:clutter+clutter/conform / actor-size
mutter:clutter+clutter/conform / text
mutter:clutter+clutter/conform / binding-pool
mutter:clutter+clutter/conform / color
mutter:clutter+clutter/conform / interval
mutter:clutter+clutter/conform / script-parser
mutter:clutter+clutter/conform / units
mutter:clutter+clutter/conform / behaviours
mutter:clutter+clutter/conform / group
mutter:clutter+clutter/conform / rectangle
mutter:clutter+clutter/conform / texture
mutter:core+mutter/stacking / basic-x11
mutter:core+mutter/stacking / basic-wayland
mutter:core+mutter/stacking / client-side-decorated
mutter:core+mutter/stacking / closed-transient
mutter:core+mutter/stacking / minimized
mutter:core+mutter/stacking / mixed-windows
mutter:core+mutter/stacking / set-parent
mutter:core+mutter/stacking / override-redirect
mutter:core+mutter/stacking / set-parent-exported
mutter:core+mutter/unit / normal
mutter:core+mutter/unit / headless-start