libpng: Improve ptest

Install libpng test-suite to run it as a ptest.
As the test-suite takes more than 30 seconds to run,
add libpng-ptest to PTESTS_SLOW in ptest-packagelists.inc

(From OE-Core rev: 5835b803acc255c227213670588dd01c5168c266)

Signed-off-by: Poonam Jadhav <poonam.jadhav@kpit.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

meta/conf/distro/include/ptest-packagelists.inc

@@ -29,7 +29,6 @@ PTESTS_FAST = "\
     libnl-ptest \
     libmodule-build-perl-ptest \
     libpcre-ptest \
-    libpng-ptest \
     libssh2-ptest \
     libtimedate-perl-ptest \
     libtest-needs-perl-ptest \
@@ -88,6 +87,7 @@ PTESTS_SLOW = "\
     glib-2.0-ptest \
     gstreamer1.0-ptest \
     libevent-ptest \
+    libpng-ptest \
     lttng-tools-ptest \
     openssh-ptest \
     openssl-ptest \