Fix failing tests

[pditommaso]

Some tests are failing because a repository on ECR has been deleted

should retrieve image digest on ECR

resp1 == 'sha256:05f9dc67e6ec879773de726b800d4d5044f8bd8e67da728484fbdea56af1fdff'
|     |
null  false

	at io.seqera.wave.core.RegistryProxyServiceTest.should retrieve image digest on ECR(RegistryProxyServiceTest.groovy:69)

should call head manifest on amazon

resp.statusCode() == 200
|    |            |
|    404          false
(HEAD https://xxx.dkr.ecr.eu-west-1.amazonaws.com/v2/wave/kaniko/manifests/0.1.0) 404

	at io.seqera.wave.proxy.ProxyClientTest.should call head manifest on amazon(ProxyClientTest.groovy:184)

should call target manifest on amazon

resp.statusCode() == 200
|    |            |
|    404          false
(GET https://xxx.dkr.ecr.eu-west-1.amazonaws.com/v2/wave/kaniko/manifests/0.1.0) 404

	at io.seqera.wave.proxy.ProxyClientTest.should call target manifest on amazon(ProxyClientTest.groovy:163)