MDEV-20787: Script dgcov.pl does not workbb-10.2-anel-MDEV-20787

Let's change CMakeList with `--coverage` flag as an alias for
`-fprofile-arcs -ftest-coverage -lgcov` in addition.
When the server is compiled with `-DENABLE_GCOV=ON`, from object files are generated
`.gcno` and `.gcda` files.
`./mtr --gcov is_check_constraint` is invoking the script calls
`./dgcov.pl --purge`, `./mtr is_check_constraint`,
`./dgcov.pl --generate>/var/last_changes.dgcov`.
The `purge` flag is clearing `.gcda` files (and others extensions),
while running the test new `.gcda` files are obtained.
With `generate` flag, `gcov -i` (`intermediate format`) is called
on obtained `<object-files-name>.gcda` files (`dbug.c.gcda` e.g.).
The patch is tested on `gcov 6.3` and `gcov 7.4` versions
and can be seen that resulting `.gcov` file for `6.3` creates
`<full path>.gcov` (`dbug.c.gcda.gcov` e.g.) file,
where `gcov 7.4` is still creating `object-file-names.gcov`(`dbug.c.gcov`) files
as `gcov` in general is doing.

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