Magento 2.1 – How to Add Custom Button on Custom Grid in Customer View Adminhtml

I think your problem is that the <blocks> tag from your config.xml is not in the right place.
It should be a child of the node <global>

So make your config.xml look like this:

<?xml version="1.0" ?>
<config>
  <modules>
    <CPT_TCS>
      <version>
        1.0.0
      </version>
    </CPT_TCS>
  </modules>
  <global>
  <blocks>
    <cpttcs>
      <class>CPT_TCS_Block</class>
    </cpttcs>
    <adminhtml>
      <rewrite>
        <sales_order_view>CPT_TCS_Block_Adminhtml_Sales_Order_View</sales_order_view>
      </rewrite>
    </adminhtml>
  </blocks>
  </global>
</config>

Alternative
You don't need to rewrite the order view block to add a button. you can do it cleaner with an observer.
Add this in the config.xml of your module under the config root note.

<adminhtml>
    <events>
        <controller_action_layout_render_before_adminhtml_sales_order_view>
            <observers>
                <ctp_tcs>
                    <class>ctptcs/adminhtml_observer</class>
                    <method>addTcsButton</method>
                </ctp_tcs>
            </observers>
        </controller_action_layout_render_before_adminhtml_sales_order_view>
    </events>
</adminhtml>

Then declare your models in the same config.xml.
Under the <global> tag add this:

<models>
    <cpttcs>
        <class>CPT_TCS_Model</class>
    </cpttcs>
</models>

Then create the observer class CTP/TCS/Model/Adminhtml/Observer.php

<?php 
class CTP_TCS_Model_Adminhtml_Observer 
{
    public function addTcsButton($observer) {
        $block = Mage::app()->getLayout()->getBlock('sales_order_edit');
        if (!$block){
            return $this;
        }
        $order = Mage::registry('current_order');
        $url = Mage::helper("adminhtml")->getUrl(
            "CPT_TCS/order/doSomething",
            array('order_id'=>$order->getId())
        );
        $block->addButton('cygtest_resubmit', array(
                'label'     => Mage::helper('sales')->__('Send TCS'),
                'onclick'   => 'setLocation(\'' . $url . '\')',
                'class'     => 'go'
        ));
        return $this;
    }
}

I think you can extend Magento/Ui/view/base/web/templates/grid/cells/actions.html. Second solution, you can add a new column action for test like

<actionsColumn name="test_actions" class="Space\YourModule\Ui\Component\Listing\Column\TestAction">
    <argument name="data" xsi:type="array">
        <item name="config" xsi:type="array">
            <item name="bodyTmpl" xsi:type="string">ui/grid/cells/html</item>
            <item name="indexField" xsi:type="string">id</item>
            <item name="sortable" xsi:type="boolean">false</item>
            <item name="label" xsi:type="string" translate="true">Test Action</item>
            <item name="urlEntityParamName" xsi:type="string">id</item>
        </item>
    </argument>
</actionsColumn>

And with class Space\YourModule\Ui\Component\Listing\Column\TestAction in prepareDataSource, we have :

$url = $this->urlBuilder->getUrl(
    'test_action',
    ['id' => $item['id']]
);

$html = '<a href="'. $url.'" target="_blank">
<button class="action-scalable action-default scalable">Test</button></a>';
$item[$this->getData('name')] = $html;