Add gearbox taps to vs gearbox_config.json (#11480)

Why I did it
For the change to support gearbox taps by gearbox_config.json (sonic-net/sonic-swss#2158), I need to add tests to sonic-swss/tests/test_gearbox.py to satisfy the test coverage of the change. The existing code in test_gearbox.py has already used brcm_gearbox_vs and here is to add some gearbox tap value to its gearbox_config.json, for the added tests in sonic-swss/tests/test_gearbox.py.

How I did it
How to verify it
This change itself will not affect existing code.

device/virtual/x86_64-kvm_x86_64-r0/brcm_gearbox_vs/gearbox_config.json

@@ -20,7 +20,17 @@
       "index": 0,
       "phy_id" : 1,
       "system_lanes": [200,201],
-      "line_lanes": [206]
+      "line_lanes": [206],
+      "system_tx_fir_pre2": [1,1],
+      "system_tx_fir_pre1": [-5,-5],
+      "system_tx_fir_main": [14,14],
+      "system_tx_fir_post1": [0,0],
+      "system_tx_fir_post2": [0,0],
+      "line_tx_fir_pre2": [0],
+      "line_tx_fir_pre1": [-1],
+      "line_tx_fir_main": [13],
+      "line_tx_fir_post1": [-5],
+      "line_tx_fir_post2": [0]
     },
     {
       "name": "Ethernet4",