Add a test for FastGaussian

I had this written, but forgot to commit it before.

Change-Id: I0e482f2bc2b90b92875f6dffa63471ebd52e6cb3
diff --git a/y2020/vision/sift/BUILD b/y2020/vision/sift/BUILD
index ec06126..5b99411 100644
--- a/y2020/vision/sift/BUILD
+++ b/y2020/vision/sift/BUILD
@@ -152,6 +152,22 @@
     ],
 )
 
+cc_test(
+    name = "fast_gaussian_test",
+    srcs = [
+        "fast_gaussian_test.cc",
+    ],
+    restricted_to = [
+        "//tools:k8",
+        "//tools:armhf-debian",
+    ],
+    deps = [
+        ":fast_gaussian",
+        "//aos/testing:googletest",
+        "//third_party:opencv",
+    ],
+)
+
 cc_binary(
     name = "testing_sift",
     srcs = [