src/common_test.c: Add some tests for files in src/common.c.
utils_vl_lookup_test: Fix dependencies.
src/utils_avltree_test.c: Add partial check for the AVL tree.
Move tests and mocks to src/tests/{,mock/}.
Tests: Make the self-rolled test framework a bit more useful and consistent.
utils_heap_test: Add some tests for the heap structure.
Add tests for src/utils_mount.[ch].

Github: #412
src/utils_mount.c: Fix wrong behavior in cu_mount_getoptionvalue().

Github: #412