Loader: refactor loader tests to avoid code duplication

Most of the loaders are tested but are reusing some duplicated code. Improve and remove such duplication.

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Migrated from T1238 (view on Phabricator)

mentioned in commit swh-loader-mercurial@c0370c6b

mentioned in commit 38279b6a

mentioned in merge request !4 (closed)

mentioned in merge request swh-loader-dir!4 (closed)

mentioned in merge request swh-loader-tar!4 (closed)

mentioned in merge request swh-loader-svn!6 (closed)

mentioned in merge request swh-loader-pypi!16 (closed)

assigned to @ardumont

added Core Loader priority:Normal labels

mentioned in commit swh-loader-svn@a89723f2

mentioned in commit swh-loader-dir@4a7ca952

mentioned in commit e29dc38d

mentioned in commit swh-loader-tar@d901f602

mentioned in commit swh-loader-pypi@e5caa83e

changed title from Loader: Refactor the loader tests to reuse to Loader: refactor loader tests to avoid code duplication

closed