Fix RunPluginEndToEndTest util

[andresgomezfrr]

Tracking issue

Closes #4343

Describe your changes

Remove duplicated mock without Run that produces that the output file is not saved to the inmemory store.

time="2023-11-01T10:34:44+01:00" level=error msg="Failed to read from the raw store [fake://bucket/prefix/2g6/outputs.pb] Error: file does not exist"
        	Error:      	Received unexpected error:
        	            	file does not exist
        	            	path:fake://bucket/prefix/2g6/outputs.pb
        	            	github.com/flyteorg/flyte/flytestdlib/storage.DefaultProtobufStore.ReadProtobuf
        	            		/Users/andresg/go/pkg/mod/github.com/flyteorg/flyte/[email protected]/storage/protobuf_store.go:39
        	            	github.com/flyteorg/flyte/flyteplugins/tests.RunPluginEndToEndTest
        	            		/Users/andresg/go/pkg/mod/github.com/flyteorg/flyte/[email protected]/tests/end_to_end.go:261
....

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• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

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