Move black-box fuzz target into fuzzers group

This makes it possible to build the fuzzer in Chromium's asan builder
group by having it depend on the tint fuzzers group, and means that the
logic for when particular fuzz targets are built remains encapsulated in
the tint fuzzer build rules.

Change-Id: Ic8d6131ccf1759a25fc3d736ae507cd173931616
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/64181
Auto-Submit: Alastair Donaldson <afdx@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

fuzzers/BUILD.gn

@@ -336,6 +336,11 @@ if (build_with_chromium) {
         ":tint_wgsl_reader_msl_writer_fuzzer",
       ]
     }
+    if (tint_build_wgsl_reader && tint_build_hlsl_writer &&
+        tint_build_msl_writer && tint_build_spv_writer &&
+        tint_build_wgsl_writer) {
+      deps += [ ":tint_black_box_fuzz_target" ]
+    }
   }
 } else {
   group("fuzzers") {