Capture --shm_base at event loop construction time

This sets us up better to run multi-node tests in separate /dev/shm's
rather than trying to fake channels around to get realistic behavior.

Change-Id: Idd5cda9957a87778a61d82befa3cbdec9d0182f2
diff --git a/aos/events/shm_event_loop.h b/aos/events/shm_event_loop.h
index 8dabcb5..a204a30 100644
--- a/aos/events/shm_event_loop.h
+++ b/aos/events/shm_event_loop.h
@@ -134,6 +134,11 @@
   absl::Span<const char> GetShmFetcherPrivateMemory(
       const aos::RawFetcher *fetcher) const;
 
+  // Capture the --shm_base flag at construction time.  This makes it much
+  // easier to make different shared memory regions for doing things like
+  // multi-node tests.
+  std::string shm_base_;
+
   std::vector<std::function<void()>> on_run_;
   int priority_ = 0;
   cpu_set_t affinity_ = DefaultAffinity();