bionic tests: use GTEST_SKIP.

Also be a bit more to the point in our messages, focusing on "why" not
"what".

Test: ran tests
Change-Id: I297806c7a102bd52602dcd2fcf7a2cd34aba3a11
diff --git a/tests/signal_test.cpp b/tests/signal_test.cpp
index 77b004f..1ae174a 100644
--- a/tests/signal_test.cpp
+++ b/tests/signal_test.cpp
@@ -582,7 +582,7 @@
   ASSERT_TRUE(sys_signame[0] == nullptr);
   ASSERT_STREQ("HUP", sys_signame[SIGHUP]);
 #else
-  GTEST_LOG_(INFO) << "This test does nothing.\n";
+  GTEST_SKIP() << "glibc doesn't have sys_signame";
 #endif
 }