[modular] Disable flaky tests embed_shell and story_shell

MF-174

Test=none

Change-Id: Ia4f9bcb0cc6a661b1aec3ae3f2a93bab81a2615d
diff --git a/tests/modular_tests.json b/tests/modular_tests.json
index 055a281..fa04004 100644
--- a/tests/modular_tests.json
+++ b/tests/modular_tests.json
@@ -29,6 +29,7 @@
     },
     {
       "name": "embed_shell",
+      "disabled": true,
       "exec": [
         "basemgr",
         "--test",
@@ -94,6 +95,7 @@
     },
     {
       "name": "story_shell",
+      "disabled": true,
       "exec": [
         "fuchsia-pkg://fuchsia.com/basemgr#meta/basemgr.cmx",
         "--test",