Revert "Temporarily disable FdsanTest.vfork."

This reverts commit b4cbff351f5d3ea5aad56483a68531b59f9fc1aa.

Bug: http://b/156400255
Test: none
Change-Id: I3a936542ccff8989b4671f3e03dd9d1d5559d6ff
diff --git a/tests/fdsan_test.cpp b/tests/fdsan_test.cpp
index 9932b21..134d621 100644
--- a/tests/fdsan_test.cpp
+++ b/tests/fdsan_test.cpp
@@ -194,7 +194,7 @@
 #endif
 }
 
-TEST_F(FdsanTest, DISABLED_vfork) {
+TEST_F(FdsanTest, vfork) {
   android::base::unique_fd fd(open("/dev/null", O_RDONLY));
 
   pid_t rc = vfork();