Merge branch 'selftests-TEST_INCLUDES'

Benjamin Poirier says:

====================
selftests: Add TEST_INCLUDES directive and adjust tests to use it

After commit 25ae948b4478 ("selftests/net: add lib.sh") but before commit
2114e83381d3 ("selftests: forwarding: Avoid failures to source
net/lib.sh"), some net selftests encountered errors when they were being
exported and run. This was because the new net/lib.sh was not exported
along with the tests. The errors were crudely avoided by duplicating some
content between net/lib.sh and net/forwarding/lib.sh in 2114e83381d3.

In order to restore the sourcing of net/lib.sh from net/forwarding/lib.sh
and remove the duplicated content, this series introduces a new selftests
Makefile variable to list extra files to export from other directories and
makes use of it to avoid reintroducing the errors mentioned above.

v2:
* "selftests: Introduce Makefile variable to list shared bash scripts"
Fix rst syntax in Documentation/dev-tools/kselftest.rst (Jakub Kicinski)

v1:
* "selftests: Introduce Makefile variable to list shared bash scripts"
Changed TEST_INCLUDES to take relative paths, like other TEST_* variables.
Paths are adjusted accordingly in the subsequent patches. (Vladimir Oltean)

* selftests: bonding: Change script interpreter
  selftests: forwarding: Remove executable bits from lib.sh
Removed from this series, submitted separately.

Since commit 2114e83381d3 ("selftests: forwarding: Avoid failures to source
net/lib.sh") resolved the test errors, this version of the series is
focused on removing the duplication that was added in that commit. Directly
rebasing the series would reintroduce the problems that 2114e83381d3
avoided before fixing them again. In order to prevent such breakage partway
through the series, patches are reordered and content changed slightly but
there is no diff at the end compared with the simple rebasing approach. I
have dropped most review tags on account of this reordering.

RFC:
https://lore.kernel.org/netdev/20231222135836.992841-1-bpoirier@nvidia.com/

Link: https://lore.kernel.org/netdev/ZXu7dGj7F9Ng8iIX@Laptop-X1/
====================

Signed-off-by: David S. Miller <davem@davemloft.net>