Fix failing unit tests with ops 1.5.3

[gregory-schiano]

Due to a bad usage of "add_relation" method of Harness, the unit tests started to fail with ops 1.5.3 (which is more strict in validating data relations, see canonical/operator#786)

[github-actions]

Test coverage for 521abb0

Name           Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------
src/charm.py     233     22     70      8    88%   84->exit, 95, 99, 327->exit, 330->exit, 334->exit, 337->exit, 489-491, 529->exit, 544-566, 570-571
----------------------------------------------------------
TOTAL            233     22     70      8    88%

Static code analysis report

[main]	INFO	profile include tests: None
[main]	INFO	profile exclude tests: None
[main]	INFO	cli include tests: None
[main]	INFO	cli exclude tests: None
[main]	INFO	using config: pyproject.toml
[main]	INFO	running on Python 3.8.10
Run started:2022-10-20 12:30:02.566138

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1078
  Total lines skipped (#nosec): 0
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):

[arturo-seijas]

LGTM

[mthaddon]

LGTM, thx