Testlib: renaming variables for consistency

Environment variables are used everywhere in testilb. This environment
variables are global variables that define the way testlib behaves.
However is was confusing to use the variables because they were
inconsistent between each other, for example some variables that define
paths would have absolute paths while other would have relative paths,
making it error prone while using them.

This commit makes the environment variables more consistent by following
a name convention for each type of variable, as an example, variables
that define absolute paths follow this convention ABS_<path_name>_DIR,
while variables that define relative paths are defined like this
<path_name>_DIR.

Signed-off-by: Castulo Martinez <castulo.martinez@intel.com>

test/functional/bundleinfo/bundle-info-basic.bats

@@ -18,7 +18,7 @@ global_setup() {
 	# when swupd initializes, if the tracking directory is empty, it will
 	# mark all installed bundles as tracked, to avoid this, let's add a dummy
 	# value to the tracking directory
-	sudo touch "$STATEDIR_TRACKING"/dummy
+	sudo touch "$ABS_TRACKING_DIR"/dummy
 
 }