sigchain: add test.

Add a gtest for the various sigchain wrappers.

Bug: http://b/73344857
Test: adb shell /data/nativetest64/art/arm64/art_sigchain_tests/sigchain_test
      (with several actual failures)
Change-Id: I331da4f44008984723e2e2733e2a6dd66658e704
diff --git a/build/Android.gtest.mk b/build/Android.gtest.mk
index 4a35ccf..388ff06 100644
--- a/build/Android.gtest.mk
+++ b/build/Android.gtest.mk
@@ -339,6 +339,7 @@
     art_profman_tests \
     art_runtime_tests \
     art_runtime_compiler_tests \
+    art_sigchain_tests \
 
 ART_TARGET_GTEST_FILES := $(foreach m,$(ART_TEST_MODULES),\
     $(ART_TEST_LIST_device_$(TARGET_ARCH)_$(m)))