Add unit tests to the pktline package

This package was added in !893 (merged). It only has indirect test coverage. Its methods are exercised in the course of other tests. We should have tests in the package itself.

Current coverage:

gitlab.com/gitlab-org/gitaly/internal/git/pktline/pktline.go:20:				NewScanner					100.0%
gitlab.com/gitlab-org/gitaly/internal/git/pktline/pktline.go:28:				Data						100.0%
gitlab.com/gitlab-org/gitaly/internal/git/pktline/pktline.go:37:				IsFlush						100.0%
gitlab.com/gitlab-org/gitaly/internal/git/pktline/pktline.go:42:				WriteString					75.0%
gitlab.com/gitlab-org/gitaly/internal/git/pktline/pktline.go:52:				WriteFlush					100.0%
gitlab.com/gitlab-org/gitaly/internal/git/pktline/pktline.go:57:				pktLineSplitter					88.2%

Which is not bad but it should come from test files in the package itself.