My Custom product tab not displaying in Magento 2

Try this in your layout file

<referenceBlock name="product.info.details">

    <block class="Magento\Catalog\Block\Product\View" name="mycustom.tab" as="mycustom" template="Test_ProductTabs::kit.phtml" group="detailed_info">
        <arguments>
            <argument translate="true" name="title" xsi:type="string">Kit Components</argument>
        </arguments>
    </block>

</referenceBlock>

product related details all are available in Magento\Catalog\Block\Product\View so no need to create custom block for get product details.

update

wrong layout file location! it should be

Test/ProductTabs/view/frontend/layout/catalog_product_view.xml

You code is fine just simple mistake you have done two mistake

please change file name

catalog_product_view_.xml

to

catalog_product_view.xml

change frontend layout and tempplate file location

Test/ProductTabs/view/layout/catalog_product_view_.xml

to

Test/ProductTabs/view/frontend/layout/catalog_product_view.xml

change template file location

Test/ProductTabs/view/templates/kit.phtml

to

Test/ProductTabs/view/frontend/templates/kit.phtml

I think the better way to do this is to follow the same approach Magento uses to add the description attribute as one of the tabs on the product details page.

First create your attribute, lets say it's tech_spec and add it to whichever attribute sets you want it to so I can be updated from the Manage Manage Products page.

Then create/update your app\code[NameSpace][ModuleName]\view\frontend\layout\catalog_product_view.xml file with:

<referenceBlock name="product.info.details">
    <block class="Magento\Catalog\Block\Product\View\Description" name="product.info.tech.spec" template="product/view/attribute.phtml" group="detailed_info">
        <arguments>
            <argument name="at_call" xsi:type="string">getTechSpec</argument>
            <argument name="at_code" xsi:type="string">tech_spec</argument>
            <argument name="css_class" xsi:type="string">tech-spec</argument>
            <argument name="at_label" xsi:type="string">none</argument>
            <argument name="title" translate="true" xsi:type="string">Technical Specification</argument>
        </arguments>
    </block>
</referenceBlock>

---

Alternatively if you don't want to add an attribute you can add a template instead.

<referenceBlock name="product.info.details">
    <block class="Magento\Catalog\Block\Product\View" name="product.info.tech.spec" template="Magento_Catalog::product/view/tech-spec.phtml" group="detailed_info" >
        <arguments>
            <argument translate="true" name="title" xsi:type="string">Technical Specification</argument>
        </arguments>
    </block>
</referenceBlock>