Don't allow multiple initdb processes on OSX.

Fixes:
[initdb:pid(95804)] INFO  i.z.t.d.p.embedded.EmbeddedPostgres - 2020-09-17 12:59:51.996 MDT [95916] DETAIL:  Failed system call was shmget(key=5432002, size=56, 03600).

Author: jameshilliard

src/main/java/io/zonky/test/db/postgres/embedded/EmbeddedPostgres.java

@@ -237,6 +237,8 @@ private void lock() throws IOException
         }
     }
 
+    private static final Lock INITDB_LOCK = new ReentrantLock();
+
     private void initdb()
     {
         final StopWatch watch = new StopWatch();
@@ -246,7 +248,16 @@ private void initdb()
                 "-A", "trust", "-U", PG_SUPERUSER,
                 "-D", dataDirectory.getPath(), "-E", "UTF-8"));
         args.addAll(createLocaleOptions());
-        system(INIT_DB, args);
+        if (SystemUtils.IS_OS_MAC_OSX) {
+            INITDB_LOCK.lock();
+            try {
+                system(INIT_DB, args);
+            } finally {
+                INITDB_LOCK.unlock();
+            }
+        } else {
+            system(INIT_DB, args);
+        }
         LOG.info("{} initdb completed in {}", instanceId, watch);
     }