tests: replace ok(ret == KNOT_E???) with is_int(KNOT_E???, ret) to ease debugging

Throwing away unexpected retun codes makes debugging harder.
TAP function is_int can nicely log mismatches which makes debugging easier.

To make our lives easier I replaced all ok(ret == KNOT_E???) checks with
is_int() calls. It was done using following command
+ manual review and reverts in libknot/test_control.c.

$ find -name '*.c' | xargs sed -i 's#ok(ret == \(KNOT_E[0-9A-Z_]\+\),#is_int(\1, ret,#'