chore: Update .gitignore

This is a cherry-pick of commit df3b9173277aae60b08a216dc23484f6ec171ef5
from branch 'libpng18'.

.gitignore

@@ -122,6 +122,7 @@ ipch/
 *.out
 *out.png
 [._]deps/
+[._]libs/
 .dirstamp
 /Makefile
 /autom4te.cache/
@@ -143,13 +144,16 @@ pnglibconf.dfn
 pnglibconf.pre
 pngprefix.h
 
-# Libpng test programs
+# Libpng test programs, scripts and artifacts
-png-fix-itxt
+/test-suite.log
-pngcp
+/tests/*.log
-pngfix
+/tests/*.trs
-pngimage
+/png-fix-itxt
-pngstest
+/pngcp
-pngtest
+/pngfix
-pngunknown
+/pngimage
-pngvalid
+/pngstest
-timepng
+/pngtest
+/pngunknown
+/pngvalid
+/timepng