[1213] Force git to keep nsx2 directories in the build tree

tests/system/ixfr/in-1/nsx2/README

@@ -0,0 +1,3 @@
+This directory should be empty. A README file is placed here to ensure git
+notes the directory's presence.  It can be removed if other files are placed
+here.

tests/system/ixfr/in-2/nsx2/README

@@ -0,0 +1,3 @@
+This directory should be empty. A README file is placed here to ensure git
+notes the directory's presence.  It can be removed if other files are placed
+here.

tests/system/ixfr/in-3/nsx2/README

@@ -0,0 +1,3 @@
+This directory should be empty. A README file is placed here to ensure git
+notes the directory's presence.  It can be removed if other files are placed
+here.

tests/system/ixfr/in-4/nsx2/README

@@ -0,0 +1,3 @@
+This directory should be empty. A README file is placed here to ensure git
+notes the directory's presence.  It can be removed if other files are placed
+here.