add test for export env var

tests/test_vars.sh

+#!/bin/sh
+
+check()
+{
+        [ "$2" != "$3" ] && echo "FATAL with '$1': result '$3' do not match with '$2'" || echo "OK for '$1' with '$2'"
+}
+
+func()
+{
+	echo "$VAR"
+}
+
+VAR=
+check VAR-empty "" $(func)
+VAR=test
+check VAR-fill "test" $(func)
+
+VAR=
+check VAR-setbefore "again" $(VAR=again func)
+check VAR-emptyafter "" $(func)
+
+# stranges only for function
+VAR=try func
+check VAR-emptyaftertry "" $(func)
+
+VAR=
+VAR=try true
+check VAR-emptyaftertry2 "" $(func)