[gcmole] Include gcmole-test.cc for bots

The bots currently fail to run the gcmole self tests, because the file
is not contained in the generated archive.
This CL fixes that.

R=mstarzinger@chromium.org
CC=mslekova@chromium.org

Change-Id: I691c207be1809516a5cc5e250287427674146a7e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1789707Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63608}

tools/gcmole/BUILD.gn

@@ -11,6 +11,7 @@ group("v8_run_gcmole") {
     "gccause.lua",
     "GCMOLE.gn",
     "gcmole.lua",
+    "gcmole-test.cc",
     "gcmole-tools/",
     "parallel.py",
     "run-gcmole.py",