cleanup: Map test files according to structure in src

For example cmd_account() is located in src/command/cmd_funcs.c.
The unit test was located in tests/unittests/test_cmd_account.c.
Let's move it to a subdirectory like tests/unittests/command/test_cmd_account.c
to correpsond to the same structure.

tests/unittests/plugins/test_callbacks.h

@@ -0,0 +1,2 @@
+void returns_no_commands(void** state);
+void returns_commands(void** state);