Passing global var to QuickCheck statemachine

The data_dir used by the tests is given at runtime. This has as a
disadvantage that the generate test has a hardcoded data_dir in it
(ssh_eqc_client_server_dirs below):

[{set,{var,1},
{call,ssh_eqc_client_server,initial_state,
[{state,false,[],[],[],[],"ssh_eqc_client_server_dirs"}]}},
{set,{var,2},
{call,ssh_eqc_client_server,ssh_server,
[{{127,1,1,1},
{call,ssh_eqc_client_server,inet_port,[{127,1,1,1}]}},
"ssh_eqc_client_server_dirs",
[{parallel_login,true}]]}},

Re-running this tests on another machine works, since the path is
relative, but if it were absolute, it would have been hard.

Instead, we may use a symbolic representation of the data_dir and fill
it in each time one runs the property, thus even when one does a check
or recheck.

The key to this is to use a variable in the test and bind the variable
in the place where one runs the commands by using the environment
variable feature of run_commands.