liblog: run liblog#enoent only as root

Previously this test would rely on ro.debuggable being true, but there
are now some circumstances where ro.debuggable is true, yet `su` does
not exist.  Therefore we only run this test when it is root.

Also simplify some of the test macros.

Bug: 140446213
Test: passes as root, skips as non-root
Change-Id: I0651963d5c7d4f5308989ab6b04aab1042094e5a