Adapt integration tests to reuse the packed charm

[arturo-seijas]

Adapt the integration tests to reuse the exisintg charm artifact as per canonical/operator-workflows#162

[github-actions]

Test coverage for 3c4acd1

Name           Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------
src/charm.py     190      0     68      1    99%   306->309
----------------------------------------------------------
TOTAL            190      0     68      1    99%

Static code analysis report

Run started:2023-07-21 09:56:14.109036

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1133
  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):

[gtrkiller]

LGTM