Source (do not run) TF_SETUP and TF_CLEANUP

Source the setup/cleanup code so that it can be used to prepare
environment.

Disadvantage is that the files will need to be in Bash (ie. effect of
commit 4eb772d is negated), but that should not be problem as the use
of subtest.sh already implies that Bash is used.

After all, one can always easily run other program from these files.

test/README.md

 In short, it will
 
  *  take each enumerated subtest from `tf_enum_subtests`,
- *  run TF_SETUP, if such file is found,
+ *  source TF_SETUP, if such file is found,
  *  translate te subtest name to a command,
  *  launch the command,
  *  run TF_CLEANUP, if such file is found,
 ### Setup and cleanup ###
 
 Special files *TF_SETUP* and *TF_CLEANUP* (one of them or both) can be
-added along with *TF_RUN*.  These will be execued before (*TF_SETUP*)
+added along with *TF_RUN*.  These will be sourced before (*TF_SETUP*)
 and after every subtest (*TF_CLEANUP*).
 
 First, if any of these files are missing, it is considered as if the

test/include/subtest.sh

     local tcmd=""           # test command
     local setup=true        # setup function
     local cleanup=true      # cleanup command
-    test -f TF_SETUP   && setup="./TF_SETUP"
-    test -f TF_CLEANUP && cleanup="./TF_CLEANUP"
+    test -f TF_SETUP   && setup=". TF_SETUP"
+    test -f TF_CLEANUP && cleanup=". TF_CLEANUP"
     if $setup;
     then
         tcmd="$(tf_name2cmd $subtname)"