tests: properly check variables were defined

When test suite is installed some vars are not defined.

test/api/pytest.sh

@@ -30,7 +30,7 @@ aux prepare_dmeventd
 # gdb -ex r --args python FULL_PATH/lvm2/test/api/python_lvm_unit.py -v TestLvm.test_lv_active_inactive
 
 #Locate the python binding library to use.
-if [[ -n "$abs_top_builddir" ]]; then
+if [[ -n "${abs_top_builddir+varset}" ]]; then
   python_lib=($(find "$abs_top_builddir" -name lvm.so))
   if [[ ${#python_lib[*]} -ne 1 ]]; then
     if [[ ${#python_lib[*]} -gt 1 ]]; then

test/lib/inittest.sh

@@ -91,7 +91,7 @@ cd "$TESTDIR"
 mkdir lib
 
 # Setting up symlink from $i to $TESTDIR/lib
-test -n "$abs_top_builddir" && \
+test -n "${abs_top_builddir+varset}" && \
     find "$abs_top_builddir/daemons/dmeventd/plugins/" -name '*.so' \
     -exec ln -s -t lib "{}" +
 find "$TESTOLDPWD/lib" ! \( -name '*.sh' -o -name '*.[cdo]' \