logd: add a test that logd ignores SIGPIPE

Now that libsysutils sockets do not ignore SIGPIPE, logd itself must
ignore it and we therefore add a test that it is ignored properly.

Test: this test passes
Test: this test fails if logd doesn't ignore SIGPIPE
Change-Id: I65474967f40291a7abd11cfd279c8cde2d2bca14