test: Rearrange tests in order of increasing runtime

Making short tests run first is convenient to catch obvious bugs
early.
MIPS: Fix exported symbols in public API.
mmx: Add nearest over_8888_n_8888

lowlevel-blt-bench -n, over_8888_n_8888, 15 iterations on Loongson 2f:

           Before          After
          Mean StdDev     Mean StdDev   Change
    L1     9.7   0.01     19.2   0.02   +98.2%
    L2     9.6   0.11     19.2   0.16   +99.5%
    M      7.3   0.02     12.5   0.01   +72.0%
    HT     6.6   0.01     13.4   0.02  +103.2%
    VT     6.4   0.01     12.6   0.03   +96.1%
    R      6.3   0.01     11.2   0.01   +76.5%
    RT     4.4   0.01      8.1   0.03   +82.6%
mmx: Add nearest over_8888_8888

lowlevel-blt-bench -n, over_8888_8888, 15 iterations on Loongson 2f:

           Before          After
          Mean StdDev     Mean StdDev   Change
    L1    15.8   0.02     24.0   0.06   +52.0%
    L2    14.8   0.15     23.3   0.13   +56.9%
    M     10.3   0.01     13.8   0.03   +33.6%
    HT    10.0   0.02     14.5   0.05   +44.7%
    VT     9.7   0.02     13.5   0.04   +39.2%
    R      9.1   0.01     12.2   0.04   +34.4%
    RT     7.1   0.06      8.9   0.09   +25.2%
Fix comment about BILINEAR_INTERPOLATION_BITS to say < 8 rather than <= 8

Since a4c79d695d52c94647b1aff7 the constant
BILINEAR_INTERPOLATION_BITS must be strictly less than 8, so fix the
comment to say this, and also add a COMPILE_TIME_ASSERT in the
bilinear fetcher in pixman-fast-path.c
mmx: Fix _mm_empty problems for over_8888_8888/over_8888_n_8888

Using "--disable-sse2 --disable-ssse3" configure options and
CFLAGS="-m32 -O2 -g" on an x86 system results in pixman "make check"
failures:

    ../test-driver: line 95: 29874 Aborted
    FAIL: affine-test
    ../test-driver: line 95: 29887 Aborted
    FAIL: scaling-test

One _mm_empty () was missing and another one is needed to workaround
an old GCC bug https://gcc.gnu.org/PR47759 (GCC may move MMX instructions
around and cause test suite failures).

Reviewed-by: Matt Turner <mattst88@gmail.com>