Fixed test-order dependency in gmxpreprocess tests.
Due to the use of static variable in genconf.cpp and
solvate.cpp, used for initializing command-line args
the result of the tests were dependent on the order of
execution. The reason is that static variables set in one
of the test runs are "inherited" by the next run. A
particular example is a randum number seed, which when 0
is set to a random seed, but just the first time around. This
is obviously not a good idea when aiming for reproducability.
Part of #2113.
Change-Id: I8bd8d36c75dcd20824e7f3db7268512387cd04ca