You can add portlet in control panel by adding below lines in your
<control-panel-entry-category>content</control-panel-entry-category>
<control-panel-entry-weight> 9.5</control-panel-entry-weight>
So your liferay-portlet.xml will look like
<liferay-portlet-app>
<portlet>
<portlet-name>Test Portlet</portlet-name>
<icon>/icon.png</icon>
<control-panel-entry-category>content</control-panel-entry-category>
<control-panel-entry-weight> 9.5</control-panel-entry-weight>
<instanceable>false</instanceable>
<header-portlet-css>/css/main.css</header-portlet-css>
<footer-portlet-javascript> /js/main.js
</footer-portlet-javascript>
<css-class-wrapper>
test-portlet
</css-class-wrapper>
<add-default-resource>true</add-default-resource>
</portlet>
<role-mapper>
<role-name>administrator</role-name>
<role-link>Administrator</role-link>
</role-mapper>
<role-mapper>
<role-name>guest</role-name>
<role-link>Guest</role-link>
</role-mapper>
<role-mapper>
<role-name>power-user</role-name>
<role-link>Power User</role-link>
</role-mapper>
<role-mapper>
<role-name>user</role-name>
<role-link>User</role-link>
</role-mapper>
</liferay-portlet-app>
Let me know if it worked for you.