Add Test plan issue template

.gitlab/issue_templates/Test plan.md

+# Test Plan
+
+<!-- This issue outlines testing activities related to a particular issue or epic.
+
+[Here is an example test plan](https://gitlab.com/gitlab-org/gitlab-ce/issues/50353)
+
+This and other comments should be removed as you write the plan -->
+
+## Introduction
+
+<!-- Briefly outline what is being tested
+
+Mention the issue(s) this test plan is related to -->
+
+## Scope
+
+<!-- State any limits on aspects of the feature being tested
+Outline the types of data to be included
+Outline the types of tests to be performed (functional, security, performance,
+database, automated, etc) -->
+
+## ACC Matrix
+
+<!-- Use the matrix below as a template to identify the Attributes, Components, and
+Capabilities relevant to the scope of this test plan. Add or remove Attributes
+and Components as required and list Capabilities in the next section
+
+Attributes (columns) are adverbs or adjectives that describe (at a high level)
+the qualities testing is meant to ensure Components have.
+
+Components (rows) are nouns that define major parts of the product being tested.
+
+Capabilities link Attributes and Components. They are what your product needs to
+do to make sure a Component fulfills an Attribute
+
+For more information see the [Google Testing Blog article about the 10 minute
+test plan](https://testing.googleblog.com/2011/09/10-minute-test-plan.html) and
+[this wiki page from an open-source tool that implements the ACC
+model](https://code.google.com/archive/p/test-analytics/wikis/AccExplained.wiki). -->
+
+|            | Simple | Secure | Responsive | Obvious | Stable |
+|------------|:------:|:------:|:----------:|:-------:|:------:|
+| Admin      |        |        |            |         |        |
+| Groups     |        |        |            |         |        |
+| Project    |        |        |            |         |        |
+| Repository |        |        |            |         |        |
+| Issues     |        |        |            |         |        |
+| MRs        |        |        |            |         |        |
+| CI/CD      |        |        |            |         |        |
+| Ops        |        |        |            |         |        |
+| Registry   |        |        |            |         |        |
+| Wiki       |        |        |            |         |        |
+| Snippets   |        |        |            |         |        |
+| Settings   |        |        |            |         |        |
+| Tracking   |        |        |            |         |        |
+| API        |        |        |            |         |        |
+
+## Capabilities
+
+<!-- Use the ACC matrix above to help you identify Capabilities at each relevant
+intersection of Components and Attributes.
+
+Some features might be simple enough that they only involve one Component, while
+more complex features could involve multiple or even all.
+
+Example (from https://gitlab.com/gitlab-org/gitlab-ce/issues/50353):
+* Respository is
+  * Simple
+    * It's easy to select the desired file template
+    * It doesn't require unnecessary actions to save the change
+    * It's easy to undo the change after selecting a template
+  * Responsive
+    * The list of templates can be restricted to allow a user to find a specific template among many
+    * Once a template is selected the file content updates quickly and smoothly
+-->
+
+## Test Plan
+
+<!-- If the scope is small enough you may not need to write a list of tests to
+perform. It might be enough to use the Capabilities to guide your testing.
+
+If the feature is more complex, especially if it involves multiple Components,
+briefly outline a set of tests here. When identifying tests to perform be sure
+to consider risk. Note inherent/known levels of risk so that testing can focus
+on high risk areas first.
+
+New end-to-end and integration tests (Selenium and API) should be added to the
+[Test Coverage sheet](https://docs.google.com/spreadsheets/d/1RlLfXGboJmNVIPP9jgFV5sXIACGfdcFq1tKd7xnlb74/)
+
+Please note if automated tests already exist.
+
+When adding new automated tests, please keep [testing levels](https://docs.gitlab.com/ce/development/testing_guide/testing_levels.html)
+in mind.
+-->
+
+/label ~Quality
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