icon-theme: Test inheritance vs. generic fallback

Add a few tests that check the behavior of icon theme inheritance
vs generic fallback vs symbolic icons.

testsuite/gtk/icons2/index.theme

@@ -0,0 +1,12 @@
+[Icon Theme]
+Name=Icons2
+Comment=Testing of the Icon theme code
+
+Directories=scalable
+
+[scalable]
+Context=scalable icons
+Type=Scalable
+Size=128
+MinSize=1
+MaxSize=256

testsuite/gtk/icons2/scalable/one-two-symbolic-rtl.svg

@@ -0,0 +1,5 @@
+<?xml version="1.0" standalone="no"?>
+<svg width="128" height="128" version="1.1" xmlns="http://www.w3.org/2000/svg">
+  <rect x="0" y="0" width="64" height="64" fill="black"/>
+  <rect x="64" y="64" width="64" height="64" fill="black"/>
+</svg>

testsuite/gtk/icons2/scalable/one-two-symbolic.svg

@@ -0,0 +1,5 @@
+<?xml version="1.0" standalone="no"?>
+<svg width="128" height="128" version="1.1" xmlns="http://www.w3.org/2000/svg">
+  <rect x="0" y="0" width="64" height="64" fill="black"/>
+  <rect x="64" y="64" width="64" height="64" fill="black"/>
+</svg>

testsuite/gtk/icons2/scalable/one-two-three-symbolic.svg

@@ -0,0 +1,5 @@
+<?xml version="1.0" standalone="no"?>
+<svg width="128" height="128" version="1.1" xmlns="http://www.w3.org/2000/svg">
+  <rect x="0" y="0" width="64" height="64" fill="black"/>
+  <rect x="64" y="64" width="64" height="64" fill="black"/>
+</svg>