test(uffd): restore TestSerial{Missing,MissingWrite} and TestParallelMissingWriteWithPrefault loads to 1_000_000

Continues the regression sweep started in 4681ab2. The original
PR #1415 (88c3960, Nov 3 2025) introduced these tests at 1_000_000
operations. PR #1450 (6ee2ebb, Nov 7 2025, "Modify UFFD tests to
run serve loop in a separate process") cut them all to 10_000 in the
same commit that switched from in-process to cross-process tests, with
no commit message justification. The cross-process refactor itself
does not require lower iteration counts: the per-iteration work is
still in-process memory access; only the page-state snapshot RPC is
cross-process and is called once per test.

Reverts:
  - TestSerialMissing:                10_000 -> 1_000_000
  - TestSerialMissingWrite:           10_000 -> 1_000_000
  - TestParallelMissingWriteWithPrefault: 10_000 -> 1_000_000

Per-test wall time on this branch (sudo go test, no -race):
  - TestSerialMissing                  ~3s
  - TestSerialMissingWrite             ~3s
  - TestParallelMissingWriteWithPrefault ~2s

TestParallelMissingWithPrefault stays at 10_000: it was only ever
parallelOperations := 10 in #1415, then bumped to 10_000 in #1450,
so 10_000 is already above its historical baseline.

Author: ValentaTomas

packages/orchestrator/pkg/sandbox/uffd/userfaultfd/missing_test.go

@@ -218,7 +218,7 @@ func TestParallelMissingWithPrefault(t *testing.T) {
 func TestSerialMissing(t *testing.T) {
 	t.Parallel()
 
-	serialOperations := 10_000
+	serialOperations := 1_000_000
 
 	tt := testConfig{
 		pagesize:      header.PageSize,

packages/orchestrator/pkg/sandbox/uffd/userfaultfd/missing_write_test.go

@@ -173,7 +173,7 @@ func TestParallelMissingWrite(t *testing.T) {
 func TestParallelMissingWriteWithPrefault(t *testing.T) {
 	t.Parallel()
 
-	parallelOperations := 10_000
+	parallelOperations := 1_000_000
 
 	tt := testConfig{
 		pagesize:      header.PageSize,
@@ -213,7 +213,7 @@ func TestParallelMissingWriteWithPrefault(t *testing.T) {
 func TestSerialMissingWrite(t *testing.T) {
 	t.Parallel()
 
-	serialOperations := 10_000
+	serialOperations := 1_000_000
 
 	tt := testConfig{
 		pagesize:      header.PageSize,