fs-test.sh: fix pre-requisite detection

In the following snippet:

if [ ! -x `which $prereq` ]; then

When $prereq does not exist, `which $prereq` evaluates to the empty string,
which results in *no* argument being passed to the -x operator, which then
evaluates to true, which is the equivalent of the prereq having been found. In
order for this to fail as expected, we must pass an empty argument, which then
causes -x to fail. Do this by wrapping the `` in quotes so there's always an
argument to -x, even if the value of the argument is zero-length.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>