Clean case directory during preparation step

* Cleaning directory is crutial to avoid unexpected uptest behavior when
the cases are changing between uptest run
* The cases contents can change during the uptest version upgrade
  and when we use `--skip-delete` option
* Without this change the uptest behavior is confusing the user, e.g.
  the `--skip-delete` option does not effectively work after the
  standard run because `03-delete.yaml` case is not getting removed from
  the case directory

Signed-off-by: Yury Tsarev <yury@upbound.io>

internal/prepare.go

@@ -79,6 +79,9 @@ type preparer struct {
 
 func (p *preparer) prepareManifests() ([]config.Manifest, error) {
 	caseDirectory := filepath.Join(p.testDirectory, caseDirectory)
+	if err := os.RemoveAll(caseDirectory); err != nil {
+		return nil, errors.Wrapf(err, "cannot clean directory %s", caseDirectory)
+	}
 	if err := os.MkdirAll(caseDirectory, os.ModePerm); err != nil {
 		return nil, errors.Wrapf(err, "cannot create directory %s", caseDirectory)
 	}