Fix incorrect CMake test directory when building as a subproject

This commit fixes the following error when libarchive is being built as a subproject of another project.

```
[ 33%] Building C object libarchive/libarchive/test/CMakeFiles/libarchive_test.dir/main.c.o
/stuff/chenxiaolong/Android/DualBootPatcher/MultiBootPatcher/libarchive/libarchive/test/main.c:27:24: fatal error: test_utils.h: No such file or directory
 #include "test_utils.h"
                        ^
compilation terminated.
```

(The parent project would simply have a line like `add_subdirectory(libarchive)`)

This is actually just an extension of commit e800985adbb0820c5692ea016b3a8499d69a73dc, which fixed the same issue a while back :)