fixed_vector test: fix GCC warning

GCC 4.7.2 emits the following warning:

.../test/unit/numeric/blas/fixed_vector.cpp:172:9:   required from ‘void construction::test_initialiser_list() [with T = signed char]’
.../test/unit/numeric/blas/fixed_vector.cpp:164:1:   required from here
.../boost/test/test_tools.hpp:536:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]