TestDeleteContext fails when cmd/testdata directory is missing

まだ誰も着手していません。

### Describe the bug

While running the test suite with `go test ./...`, the `TestDeleteContext` test fails because it tries to write/read a config file at `./testdata/local.config`. However, the `cmd/testdata` directory does not exist in a fresh clone, causing the test to fail with:

```
open ./testdata/local.config: no such file or directory
```

This makes the test dependent on a manually existing local directory instead of being fully self-contained.

### Expected behavior

`go test ./...` should pass on a fresh clone without requiring the `cmd/testdata` directory to already exist. The test should create any temporary files or directories it needs during execution.

### Actual behavior

The test suite fails in `TestDeleteContext`:

```
--- FAIL: TestDeleteContext (0.00s)
context_test.go:44:
Error: Received unexpected error:
open ./testdata/local.config: no such file or directory
```

### How to Reproduce?

1. Clone the repository fresh: `git clone https://github.com/microcks/microcks-cli.git`
2. Change into the directory: `cd microcks-cli`
3. Run the test suite: `go test ./...`
4. Observe the failure in `cmd/context_test.go` at `TestDeleteContext`.

### Microcks version or git rev

_No response_

### Install method (`docker-compose`, `helm chart`, `operator`, `docker-desktop extension`,...)

_No response_

### Additional information

**Root cause.** `cmd/context_test.go` declares a hardcoded path:

```go
const testConfigFilePath = "./testdata/local.config"
```

and then calls `os.WriteFile(testConfigFilePath, ...)`. `os.WriteFile` does not create parent directories, and `.gitignore` excludes `**/testdata/**`, so the `cmd/testdata/` directory does not exist on a clean clone.

**Proposed fix.** Use Go's `t.TempDir()` so the test creates an isolated, auto-cleaned temporary directory on every run:

```go
testConfigFilePath := filepath.Join(t.TempDir(), "local.config")
```

This makes the test self-contained, removes the dependency on repository-local folders, and prevents state leakage between consecutive runs.

I would like to work on this issue if the maintainers agree with the proposed approach. I can send a small PR that updates the test to use `t.TempDir()`.

cmd/context_test.go の TestDeleteContext から始め、testConfigFilePath がどのように使われているかを確認します。go test ./... を実行して、cmd/testdata が存在しないことによる失敗を再現し、その後、テストが分離された一時的な場所を使用し、リポジトリ固有のディレクトリがない新しい checkout で成功することを確認します。

索引モデルが issue の本文から書いたものです。