Test aom_convolve8 with bilinear filters

The unit-tests for aom_convolve8 are only covering 4-tap and 8-tap
filters. Add coverage for bilinear (2-tap) filters as well.

Change-Id: I1bf4ab4000db66ff192fab28b08f8c43af450660
diff --git a/test/convolve_test.cc b/test/convolve_test.cc
index cab5909..41e838a 100644
--- a/test/convolve_test.cc
+++ b/test/convolve_test.cc
@@ -474,7 +474,7 @@
       ref = CONVERT_TO_BYTEPTR(ref16_);
     }
     int subpel_search;
-    for (subpel_search = USE_4_TAPS; subpel_search <= USE_8_TAPS;
+    for (subpel_search = USE_2_TAPS; subpel_search <= USE_8_TAPS;
          ++subpel_search) {
       for (int filter_bank = 0; filter_bank < kNumFilterBanks; ++filter_bank) {
         const InterpFilter filter = (InterpFilter)filter_bank;
@@ -555,7 +555,7 @@
         }
         if (axis) seed_val += 8;
         int subpel_search;
-        for (subpel_search = USE_4_TAPS; subpel_search <= USE_8_TAPS;
+        for (subpel_search = USE_2_TAPS; subpel_search <= USE_8_TAPS;
              ++subpel_search) {
           for (int filter_bank = 0; filter_bank < kNumFilterBanks;
                ++filter_bank) {
@@ -687,7 +687,7 @@
 
 void FiltersWontSaturateWhenAddedPairwise() {
   int subpel_search;
-  for (subpel_search = USE_4_TAPS; subpel_search <= USE_8_TAPS;
+  for (subpel_search = USE_2_TAPS; subpel_search <= USE_8_TAPS;
        ++subpel_search) {
     for (int filter_bank = 0; filter_bank < kNumFilterBanks; ++filter_bank) {
       const InterpFilter filter = (InterpFilter)filter_bank;