Migrate test fixtures to non-deprecated contentTypes extension point

The org.eclipse.core.runtime.contentTypes extension point is being
removed in eclipse-platform/eclipse.platform#1644. Update the two
remaining test fixtures (regression test for bug 107121 in
OpenNavigatorFolderTest, and ObjectContributionTest) to contribute to
org.eclipse.core.contenttype.contentTypes instead.

Author: vogella

tests/org.eclipse.ui.tests.performance/plugin.xml

@@ -51,7 +51,7 @@
    "regression test" for bug 107121. See OpenNavigatorFolderTest
    for more details. 
    -->
-   <extension point="org.eclipse.core.runtime.contentTypes">
+   <extension point="org.eclipse.core.contenttype.contentTypes">
    	<content-type 
    		file-extensions="htmltestonly" 
    		priority="high"

tests/org.eclipse.ui.tests/plugin.xml

@@ -1211,7 +1211,7 @@
    </extension>
 
    <extension
-   		point="org.eclipse.core.runtime.contentTypes">
+   		point="org.eclipse.core.contenttype.contentTypes">
 
       	<!-- ObjectContributionTest: This is the content-type-based object contribution -->
 		<content-type