mgr/dashboard: fix stale nvmeof spec tests from a semantic merge conflict

mgr/dashboard: fix stale nvmeof spec tests from a semantic merge conflict

mgr-dashboard-frontend-unittests has been failing on main since 2026-07-01,
unrelated to whatever PR triggers it. Bisected to cfe12305fb1, the merge
commit for PR #68180.

6182e67ea4 ("mgr/dashboard: NVMe Onboarding Setup Cards") moved gateway
group selection out of NvmeofSubsystemsComponent and
NvmeofNamespacesListComponent into a new NvmeofGatewayGroupFilterComponent,
dropping gwGroups, updateGroupSelectionState, handleGatewayGroupsError,
onGroupClear, and getSubsystems from both. Clean on its own.

PR #68180 forked from main in April, before that refactor existed, and
added three new tests against the old API. Also clean on its own, and never
rebased. Both PRs only touch lines the other doesn't, so when #68180 merged
34 minutes after 6182e67ea4, git's 3-way merge combined them without a
conflict. The merge is the only place the bug exists: new tests calling
methods the sibling parent had already deleted. TypeError at runtime,
TS2339/TS2551 from tsc, every run since. CI never caught it: GitHub was
showing #68180's make check as green from a run a full day older than
6182e67ea4.

nvmeof-namespaces-list.component.spec.ts: the dedup test is real, current
logic, it just called the old listNamespaces() instead of fetchData().
Fixed the call. The other two duplicate NvmeofGatewayGroupFilterComponent's
own spec; removed.

nvmeof-subsystems.component.spec.ts: same story, all three duplicate the
filter component's spec; removed.

Both removed error tests also checked preventDefault() on the error object,
which had no coverage elsewhere. Added that to
NvmeofGatewayGroupFilterComponent's spec, and hit a second bug while writing
it: the file's throwError(() => err) is the RxJS 7 factory overload, this
project is on RxJS 6.6.3, so it was emitting the arrow function itself as
the error. The existing error test never noticed since it only checks side
effects, not the error's identity. Used throwError(err) instead.

Verified: jest passes on all three spec files (was 6 failing), tsc -p
tsconfig.spec.json --noEmit is clean (was 17 errors), full block/nvmeof
sweep is 29/29 suites, 216/216 tests.

Fixes: cfe12305fb1
Signed-off-by: Kefu Chai <k.chai@proxmox.com>