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 48755af

Name                 Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------------
src/charm.py           201     18     41      9    89%   126-129, 157-158, 178-179, 198, 239-241, 255->266, 274-278, 352->355, 377, 390, 399->402, 477
src/file_reader.py      17      0      6      0   100%
----------------------------------------------------------------
TOTAL                  218     18     47      9    90%

Static code analysis report

Run started:2023-07-24 10:11:08.265049

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1294
  Total lines skipped (#nosec): 1
  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