testing: when using multiple yaml updaters on a file, the content is appended per update

Description

When using two updaters on a file on my unit tests, I got both changes appended (the whole doc is appended at the end) instead of replaced into the specified section in the file or yaml root.

I could only reproduce this issue when using the testing fwk. When I use my code on a file (real testing, not using the testing fwk), it works 'fine'.

Here I called over the same index file the packageUpdater and the pipelineUpdater in my unit test:

	pipelineSectionUpdater := NewPipelineSectionUpdater(func(cfg config.Configuration) ([]config.Pipeline, error) {
		pipes := cfg.Pipeline
		pipes[1].With["deps"] = "golang/go@v1.21 k8s.io/api@1.29"
		return pipes, nil
	})

	packageSectionUpdater := NewPackageSectionUpdater(func(cfg config.Configuration) (config.Package, error) {
		p := cfg.Package
		p.Epoch++
		return p, nil
	})

	s := index.Select().WhereName("blah")
	err = s.Update(packageSectionUpdater)
	require.NoError(t, err)
	err = s.Update(pipelineSectionUpdater)
	require.NoError(t, err)

	if diff := fsys.DiffAll(); diff != "" {
		t.Errorf("unexpected file modification results (-want, +got):\n%s", diff)
	}

This produced the following output:

package:
  name: blah
  version: "7"
  epoch: 1
pipeline:
  - uses: fetch
    with:
      expected-sha256: 060309d7a333d38d951bc27598c677af1796934dbd98e1024e7ad8de798fedda
      uri: https://github.com/lathiat/avahi/releases/download/v${{package.version}}/avahi-${{package.version}}.tar.gz
  - uses: go/bump
    with:
      deps: github.com/x/net@v1.0.0
  - uses: patch
    with:
      patches: CVE-2021-3468.patch
subpackages:
  - name: alsa-lib-dev
    pipeline:
      - uses: split/dev
    dependencies:
      runtime:
        - alsa-lib
# Generated by
package:
  name: blah
  version: "7"
  epoch: 1
pipeline:
  - uses: fetch
    with:
      expected-sha256: 060309d7a333d38d951bc27598c677af1796934dbd98e1024e7ad8de798fedda
      uri: https://github.com/lathiat/avahi/releases/download/v${{package.version}}/avahi-${{package.version}}.tar.gz
  - uses: go/bump
    with:
      deps: golang/go@v1.21 k8s.io/api@1.29
  - uses: patch
    with:
      patches: CVE-2021-3468.patch
subpackages:
  - name: alsa-lib-dev
    pipeline:
      - uses: split/dev
    dependencies:
      runtime:
        - alsa-lib

This is independent of the type of updaters. You can run s.Update(packageUpdater) twice and you'll get the result appended instead of just replacing the section package. When I debugged the code I found the yaml section replacement is done correctly so there must be somewhere else the error.