Fix race in tests

Contributes to #824 (closed)

Problem

--- FAIL: TestAllowedAccess (0.01s)
    receivepack_test.go:26:
    unexpected error:
    send error: EOF
    TestAllowedAccess

Solution

io.NopCloser provides a proper Close() method that does nothing, which prevents race conditions when the Gitaly client tries to close the stream.

Closes #824 (closed)