Re-enable ipv4 reassembly packetimpact tests

When these specific tests were first added, they would fail when ran by
kokoro during the presubmit tests (but they always passed locally).
These tests are now passing, so they can be re-enabled. Unclear what
changed, one hypothesis is the move from kokoro to buildkite.

Fixes #4971

PiperOrigin-RevId: 354425395

test/packetimpact/tests/ipv4_fragment_reassembly_test.go

@@ -45,8 +45,6 @@ func TestIPv4FragmentReassembly(t *testing.T) {
 		ipPayloadLen int
 		fragments    []fragmentInfo
 		expectReply  bool
-		skip         bool
-		skipReason   string
 	}{
 		{
 			description:  "basic reassembly",
@@ -78,8 +76,6 @@ func TestIPv4FragmentReassembly(t *testing.T) {
 				{offset: 2000, size: 1000, id: 7, more: 0},
 			},
 			expectReply: true,
-			skip:        true,
-			skipReason:  "gvisor.dev/issues/4971",
 		},
 		{
 			description:  "fragment subset",
@@ -91,8 +87,6 @@ func TestIPv4FragmentReassembly(t *testing.T) {
 				{offset: 2000, size: 1000, id: 8, more: 0},
 			},
 			expectReply: true,
-			skip:        true,
-			skipReason:  "gvisor.dev/issues/4971",
 		},
 		{
 			description:  "fragment overlap",
@@ -104,16 +98,10 @@ func TestIPv4FragmentReassembly(t *testing.T) {
 				{offset: 2000, size: 1000, id: 9, more: 0},
 			},
 			expectReply: false,
-			skip:        true,
-			skipReason:  "gvisor.dev/issues/4971",
 		},
 	}
 
 	for _, test := range tests {
-		if test.skip {
-			t.Skip("%s test skipped: %s", test.description, test.skipReason)
-			continue
-		}
 		t.Run(test.description, func(t *testing.T) {
 			dut := testbench.NewDUT(t)
 			conn := dut.Net.NewIPv4Conn(t, testbench.IPv4{}, testbench.IPv4{})