Simplify material test suite

- Split adjustment.mtlx into separate documents for each node.
- Remove test materials for nodes with graph definitions.

resources/Materials/TestSuite/stdlib/adjustment/luminance.mtlx

@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<materialx version="1.39">
+ <nodegraph name="luminance_color3">
+ <luminance name="luminance1" type="color3">
+ <input name="in" type="color3" value="1.0000, 0.7500, 0.5000" />
+ </luminance>
+ <output name="luminance_color3_out" type="color3" nodename="luminance1" />
+ </nodegraph>
+ <nodegraph name="luminance_color4">
+ <luminance name="luminance1" type="color4">
+ <input name="in" type="color4" value="1.0000, 0.7500, 0.5000, 0.2500" />
+ </luminance>
+ <output name="luminance_color4_out" type="color4" nodename="luminance1" />
+ </nodegraph>
+</materialx>