Potential fix for code scanning alert no. 2: Workflow does not contain permissions (#169)

This PR fixes
[AB#379924](https://dev.azure.com/EquinorASA/bb9bd8cb-74f7-4ffa-b0cb-60eff0a0be58/_workitems/edit/379924)

Potential fix for
[https://github.com/equinor/Dexpi2Imf/security/code-scanning/2](https://github.com/equinor/Dexpi2Imf/security/code-scanning/2)

To fix this issue, we should explicitly add a `permissions` block to the
workflow. This block can be added either at the workflow root or
specifically within the relevant job (in this case, `dotnet-tests`). The
permissions should be as restrictive as possible while still allowing
the workflow to function. For this workflow, the only actions observed
are checking out code, downloading a secret, setting up .NET, restoring
dependencies, building, testing, and deleting a file. None of these
require write access to repository contents or pull requests. The
minimal required permissions should be `contents: read` (to checkout
code) and allowing secrets (which do not require an entry here). Thus,
we should add:

```yaml
permissions:
  contents: read
```

at the workflow root, immediately after the `name:` and before the `on:`
block.

---


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Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>
Co-authored-by: Dag Hovland <dhovl@equinor.com>

Author: 3 people

.github/workflows/pull_request_tester.yml

@@ -1,4 +1,6 @@
 name: "Run tests (required)"
+permissions:
+  contents: read
 on:
   workflow_dispatch:
   pull_request: