test: async: tweak time multiplier

When running under valgrind, the timing is a lot slower.  Give a
longer time interval for test cases to give some cushion.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ie3e3a0d1960de2df1cea60763de8f57a3bcaa801
diff --git a/test/async/context.cpp b/test/async/context.cpp
index afae280..6e688c7 100644
--- a/test/async/context.cpp
+++ b/test/async/context.cpp
@@ -92,7 +92,7 @@
 
     EXPECT_TRUE(ran);
     EXPECT_GT(stop - start, timeout);
-    EXPECT_LT(stop - start, timeout * 2);
+    EXPECT_LT(stop - start, timeout * 3);
 }
 
 TEST_F(Context, SpawnRecursiveTask)