oeqa/selftest: Add lower layer test for overlayfs-etc

Place a test file on the /etc by means of overlayfs-user recipe.
Perform QA checks to make sure that:
- When lower layer is exposed, that it's read-only to avoid undefined behavior
- By default lower layer is not exposed

(From OE-Core rev: 2fc742178675598208b400d9889a1681249d7eea)

Signed-off-by: Vyacheslav Yurkov <v.yurkov@precitec.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-selftest/recipes-test/overlayfs-user/overlayfs-user.bb

@@ -12,6 +12,11 @@ OVERLAYFS_WRITABLE_PATHS[mnt-overlay] += "/usr/share/my-application"
 
 do_install() {
     install -d ${D}/usr/share/my-application
+    install -d ${D}${sysconfdir}
+    echo "Original file in /etc" >> ${D}${sysconfdir}/lower-layer-test.txt
 }
 
-FILES:${PN} += "/usr"
+FILES:${PN} += "\
+    ${exec_prefix} \
+    ${sysconfdir \
+"