[testsuite] Fix tiemout fail in gdb.fortran/vla-value.exp

[testsuite] Fix tiemout fail in gdb.fortran/vla-value.exp

In vla.f90, this single line of source is compiled to many instructions,

  vla2(:, :, :) = 1311                ! vla2-allocated

it is quite slow (about several minutes in my testing) to step over this
source line without range stepping.  This patch is to increase the timeout
value by 15 times, which is a magic number to make sure timeout disappears
in my testing with a slow arm-linux board.

gdb/testsuite:

2016-01-28  Yao Qi  <yao.qi@linaro.org>

	* gdb.fortran/vla-value.exp: Wrap test with with_timeout_factor.