test: increase test coverage for model critical section (>80%)

[noandrea]

➜ go test -tags test -coverprofile=/tmp/c.out . && grep -v .pb. /tmp/c.out > /tmp/coverage.out && go tool cover -html=/tmp/coverage.out -o=/tmp/coverage.html && goverreport -coverprofile=/tmp/coverage.out -sort stmt
ok  	github.com/elesto-dao/elesto/x/credential	0.031s	coverage: 2.6% of statements
+-------------------------------------------------------------+--------+---------+-------+---------+---------------+--------------+
|                            File                             | Blocks | Missing | Stmts | Missing | Block cover % | Stmt cover % |
+-------------------------------------------------------------+--------+---------+-------+---------+---------------+--------------+
| github.com/elesto-dao/elesto/x/credential/codec.go          |      2 |       2 |     2 |       2 |          0.00 |         0.00 |
| github.com/elesto-dao/elesto/x/credential/event.go          |      3 |       3 |     3 |       3 |          0.00 |         0.00 |
| github.com/elesto-dao/elesto/x/credential/msg.go            |     21 |      21 |    27 |      27 |          0.00 |         0.00 |
| github.com/elesto-dao/elesto/x/credential/credential.go     |     54 |       9 |    73 |       9 |         83.33 |        87.67 | <
| github.com/elesto-dao/elesto/x/credential/helpers.go        |      6 |       0 |     9 |       0 |        100.00 |       100.00 | <
| github.com/elesto-dao/elesto/x/credential/msg_validation.go |     21 |       0 |    21 |       0 |        100.00 |       100.00 | <
+-------------------------------------------------------------+--------+---------+-------+---------+---------------+--------------+
|                                                       Total |    107 |      35 |   135 |      41 |         67.29 |        69.63 |
+-------------------------------------------------------------+--------+---------+-------+---------+---------------+--------------+

[PaddyMc]

🙇