tests: fix bashism to load helpers.sh
`source` isn't available at POSIX sh.
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
0 files, 0 changesets, 0 total revisions
00changelog.i
requires
store
updating to branch default
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
0 files, 0 changesets, 0 total revisions
00changelog.i
branch
dirstate
hgrc
requires
store