test_slapproxy: Explain how to upgrade migration tests

Add `README.md` instructions explaining how to adapt the migration
tests after upgrading the proxy database version.

slapos/tests/test_slapproxy/README.md

+# How to update tests after database version upgrade ?
+
+You've just updated the version of the database in `slapos/proxy/schema.sql`,
+and you need to update the tests. You're in the right place!
+
+Let's call `XX` the current version number and `YY` the new version.
+Let's call `UU` the version number before `XX`.
+
+
+## Prerequisites
+
+A SlapOS node in which command `slapos` uses version `XX` of the database.
+- e.g. a Theia in which you're locally developping `slapos.core`
+
+A Python virtual environment in which the `slapos.core` egg is installed
+from the local repository with your changes, including the version upgrade
+to `YY`.
+
+For example, beside the `slapos.core` repository
+
+```
+$ python3 -m venv pyenv
+$ source pyenv/bin/activate
+(pyenv) $ cd slapos.core
+(pyenv) $ pip install -e .
+(pyenv) $ deactivate
+$
+```
+
+The command `pip install -e <path>` installs the egg from sources at `<path>`
+in editable mode, so that any change the source code is immediately taken
+into account.
+
+This provides a `slapos` command that uses version `YY` of the database
+(assuming `slapos/proxy/schema.sql` has been updated in the source code)
+whenever the virtual environment is activated.
+
+
+## In `test_slapproxy.py`
+
+- In `_MigrationTestCase`, update `current_version` from `XX` to `YY`.
+
+- Add a `TestMigrateVersionXXToLatest` test case (see the existing ones).
+  - If tables were added in version `XX`, adjust the `initial_table_list`.
+
+
+## In `test_slapproxy/`
+
+Go to `test_slapproxy/` (assuming you are at the root of `slapos.core`):
+
+```
+$ cd slapos/tests/test_slapproxy
+```
+
+With the Python virtual environment deactivated, run:
+
+```
+$ ./generate_dump.sh computer database_dump_version_UU.sql database_dump_version_XX.sql
+```
+
+This will call `slapos proxy start` with the `slapos` that uses version `XX`,
+load the `database_dump_version_UU.sql` and migrate it to version `XX`, and
+then dump it into a new file `database_dump_version_XX.sql`.
+
+Now activate the virtual environment and run:
+```
+(pyenv) $ ./generate_dump.sh slaprunner database_dump_version_current.sql database_dump_version_current.sql
+```
+
+This will call `slapos proxy start` with `slapos` from the virtual environment
+which has version `YY`, load `database_dump_version_current.sql` which is in
+version `XX`, migrate it to `YY`, and then dump it back overwriting the file.

slapos/tests/test_slapproxy/generate_dump.sh

@@ -42,12 +42,6 @@ sqlite3 ${TMPD}/proxy.db < $DUMP_BEFORE
 
 slapos proxy start --cfg ${TMPD}/slapos.cfg &
 
-# If you are running tests locally and you want to refer to the slapos being tested,
-# you can use the test python executable in this way:
-# cd ../../..
-# python -m slapos.cli.entry proxy start --cfg ${TMPD}/slapos.cfg &
-# cd -
-
 SLAPOS_PROXY_PID=$!
 
 curl --silent --retry-connrefused --retry 3 http://127.0.0.1:${PORT}/getComputerInformation?computer_id=$COMPUTER_ID