Use /bin/bash for the Flush test.

--
MOS_MIGRATED_REVID=86430394
diff --git a/src/test/java/com/google/devtools/build/lib/shell/CommandTest.java b/src/test/java/com/google/devtools/build/lib/shell/CommandTest.java
index 27d4e2a..4531cd5 100644
--- a/src/test/java/com/google/devtools/build/lib/shell/CommandTest.java
+++ b/src/test/java/com/google/devtools/build/lib/shell/CommandTest.java
@@ -475,7 +475,8 @@
   @Test
   public void testFlushing() throws Exception {
     final Command command = new Command(
-        new String[] {"/bin/sh", "-c", "echo -n Foo; sleep 0.1; echo Bar"});
+        // On darwin, /bin/sh does not support -n for the echo builtin.
+        new String[] {"/bin/bash", "-c", "echo -n Foo; sleep 0.1; echo Bar"});
     // We run this command, passing in a special output stream
     // that records when each flush() occurs.
     // We test that a flush occurs after writing "Foo"