liblog: use unique log tags for is_loggable tests

Apparently these tests are run in parallel, which causes errors since
they use the same log tag.  Use unique log tags based on pid to fix
this.

Also re-enable the previously disabled tests.

Bug: 162669552
Test: run these tests 100x+ and see that they no longer fail
Change-Id: Ib20d638e5e559bca23adec479a5dcf64075e376e