tests: add test for --variable with --env

tests/basic.sh

@@ -43,7 +43,8 @@ tests_init \
 	with_path \
 	relocatable \
 	single_depth_selectors \
-	print_variables_env
+	print_variables_env \
+	variable_env
 
 noargs_body()
 {
@@ -373,3 +374,10 @@ print_variables_env_body()
 		pkgconf --with-path=${selfdir}/lib1 --env=FOO --print-variables --cflags --libs foo
 
 }
+
+variable_env_body()
+{
+	atf_check \
+		-o inline:"FOO_INCLUDEDIR='/test/include'\n" \
+		pkgconf --with-path=${selfdir}/lib1 --env=FOO --variable=includedir foo
+}