Updated the default value in the unit test

Fixes this regression reported by Jenkins:

Error Message

expected:<java.awt.Color[r=0,g=0,b=0]> but was:<java.awt.Color[r=51,g=51,b=51]>
Stacktrace

java.lang.AssertionError: expected:<java.awt.Color[r=0,g=0,b=0]> but was:<java.awt.Color[r=51,g=51,b=51]>
	at org.junit.Assert.fail(Assert.java:88)
	at org.junit.Assert.failNotEquals(Assert.java:743)
	at org.junit.Assert.assertEquals(Assert.java:118)
	at org.junit.Assert.assertEquals(Assert.java:144)
	at org.openscience.cdk.renderer.color.CDK2DAtomColorsTest.testGetDefaultAtomColor(CDK2DAtomColorsTest.java:55)