no-up! Simplify Piglit for prototyping the port to Chrome's GPU command buffer

Transform the build system to resemble its soon-to-arrive
post-unification state. This greatly simplifies the build system, which
will make porting to Chromium's GPU command buffer easier.

Remove all tests except those in the 'OpenGL ES 2.0' group. Only the
OpenGL ES 2.0 tests will run on Chromium's GPU command buffer, anyway.
This simplifies the porting effort by removing distractions.

CMake and the piglit runner still work as usual. I verified with:
    > cmake -GNinja .
    > ninja
    > find lib bin
    | lib/
    | lib/libpiglitutil.so
    | lib/libpiglitutil_gl.so
    | bin/
    | bin/fbo_discard_gles2
    | bin/invalid-es3-queries_gles2
    | bin/link-no-vsfs_gles2
    | bin/multiple-shader-objects_gles2
    | bin/glsl-fs-pointcoord_gles2
    | bin/minmax_gles2

    > ./piglit run -n tmp tests/quick.py /tmp/results
    | [0/6]  Running Test(s): 0 1 2 3
    | [1/6] pass: 1 Running Test(s): 0 2 3 4
    | [2/6] pass: 2 Running Test(s): 0 3 4 5
    | [3/6] pass: 3 Running Test(s): 0 4 5
    | [4/6] pass: 3, skip: 1 Running Test(s): 4 5
    | [5/6] pass: 4, skip: 1 Running Test(s): 5
    | [6/6] pass: 5, skip: 1
    | Thank you for running Piglit!
    | Results have been written to /tmp/results/results.json

    > ./piglit summary console /tmp/results
    | spec/!OpenGL ES 2.0/invalid-es3-queries_gles2: skip
    | spec/!OpenGL ES 2.0/fbo_discard_gles2: pass
    | spec/!OpenGL ES 2.0/multiple-shader-objects_gles2: pass
    | spec/!OpenGL ES 2.0/glsl-fs-pointcoord: pass
    | spec/!OpenGL ES 2.0/minmax_gles2: pass
    | spec/!OpenGL ES 2.0/link-no-vsfs: pass
    | summary:
    |        pass: 5
    |        fail: 0
    |       crash: 0
    |        skip: 1
    |     timeout: 0
    |        warn: 0
    |  dmesg-warn: 0
    |  dmesg-fail: 0
    |       total: 6