Reducing precision in vector arithmetic test

The precision used was too high for POCL.
Disabling fft dimensions test for POCL

@neapel, this test fails for seemingly random sizes when run under POCL
platform. This could be a bug in POCL, but I did found (another) bug in
VexCL that was only visible in POCL.

Here is an example log:
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seed: 1378799660
1. pthread-Intel(R) Core(TM) i7 CPU         920  @ 2.67GHz (Portable
Computing Language)

Running 4 test cases...
FFT(C2C) size=55 batch=5
FFT(C2C) size=11x24 batch=1
/home/demidov/work/vexcl/tests/fft.cpp(101): error in "test_dimensions":
absolute value of rms(out, ref){0.6371322837737099} exceeds 1e-08
/home/demidov/work/vexcl/tests/fft.cpp(102): error in "test_dimensions":
absolute value of rms(back, inp){0.73121008517224417} exceeds 1e-08
FFT(C2C) size=21 batch=1
FFT(C2C) size=4 batch=1
FFT(C2C) size=22 batch=15
FFT(C2C) size=832 batch=1
FFT(C2C) size=1 batch=91
FFT(C2C) size=3x5x3x3 batch=1
/home/demidov/work/vexcl/tests/fft.cpp(101): error in "test_dimensions":
absolute value of rms(out, ref){1.2865255092966388} exceeds 1e-08
/home/demidov/work/vexcl/tests/fft.cpp(102): error in "test_dimensions":
absolute value of rms(back, inp){1.5936053040867979} exceeds 1e-08
FFT(C2C) size=208 batch=1
FFT(C2C) size=21x8 batch=1
/home/demidov/work/vexcl/tests/fft.cpp(101): error in "test_dimensions":
absolute value of rms(out, ref){0.70971232102088921} exceeds 1e-08
/home/demidov/work/vexcl/tests/fft.cpp(102): error in "test_dimensions":
absolute value of rms(back, inp){0.81701210890427911} exceeds 1e-08
FFT(C2C) size=18 batch=1
FFT(C2C) size=18x28 batch=1
/home/demidov/work/vexcl/tests/fft.cpp(101): error in "test_dimensions":
absolute value of rms(out, ref){0.54956400635471991} exceeds 1e-08
/home/demidov/work/vexcl/tests/fft.cpp(102): error in "test_dimensions":
absolute value of rms(back, inp){0.92154666873390789} exceeds 1e-08
FFT(C2C) size=495 batch=1
FFT(C2C) size=2x2x3x1x1 batch=19
/home/demidov/work/vexcl/tests/fft.cpp(101): error in "test_dimensions":
absolute value of rms(out, ref){1.4118223149235374} exceeds 1e-08
/home/demidov/work/vexcl/tests/fft.cpp(102): error in "test_dimensions":
absolute value of rms(back, inp){1.4125346294669525} exceeds 1e-08
FFT(C2C) size=350 batch=1
FFT(C2C) size=4x4x1x3x1 batch=1
/home/demidov/work/vexcl/tests/fft.cpp(101): error in "test_dimensions":
absolute value of rms(out, ref){0.9470131749938574} exceeds 1e-08
/home/demidov/work/vexcl/tests/fft.cpp(102): error in "test_dimensions":
absolute value of rms(back, inp){1.0570629457795766} exceeds 1e-08
FFT(C2C) size=1x5x3x5 batch=1
/home/demidov/work/vexcl/tests/fft.cpp(101): error in "test_dimensions":
absolute value of rms(out, ref){14.334582789062392} exceeds 1e-08
/home/demidov/work/vexcl/tests/fft.cpp(102): error in "test_dimensions":
absolute value of rms(back, inp){18.509289152264309} exceeds 1e-08
FFT(C2C) size=44 batch=8
FFT(C2C) size=1 batch=14
FFT(C2C) size=208 batch=1
FFT(C2C) size=2x3x1x1 batch=16
/home/demidov/work/vexcl/tests/fft.cpp(101): error in "test_dimensions":
absolute value of rms(out, ref){1.3671630305945559} exceeds 1e-08
/home/demidov/work/vexcl/tests/fft.cpp(102): error in "test_dimensions":
absolute value of rms(back, inp){1.3629371912069626} exceeds 1e-08
FFT(C2C) size=5x3x9 batch=1
/home/demidov/work/vexcl/tests/fft.cpp(101): error in "test_dimensions":
absolute value of rms(out, ref){1.9361168975101768} exceeds 1e-08
/home/demidov/work/vexcl/tests/fft.cpp(102): error in "test_dimensions":
absolute value of rms(back, inp){2.1009422735528753} exceeds 1e-08
FFT(C2C) size=3x7x4 batch=1
/home/demidov/work/vexcl/tests/fft.cpp(101): error in "test_dimensions":
absolute value of rms(out, ref){3.1350806455892433} exceeds 1e-08
/home/demidov/work/vexcl/tests/fft.cpp(102): error in "test_dimensions":
absolute value of rms(back, inp){3.7782236394793589} exceeds 1e-08
FFT(C2C) size=24 batch=1
FFT(C2C) size=1x20 batch=1

*** 20 failures detected in test suite "FastFourierTransform"
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