Rename test from CropNV12 to NV12Crop

When scanning profiles for unexpected _C functions
this test function contained the substring.
Changing the name will simplify the search.

TBR=harryjin@google.com
BUG=libyuv:543

Review URL: https://codereview.chromium.org/1602363002 .

unit_test/convert_test.cc

@@ -1348,7 +1348,7 @@ TEST_F(LibYUVConvertTest, MJPGToARGB) {
 
 #endif  // HAVE_JPEG
 
-TEST_F(LibYUVConvertTest, CropNV12) {
+TEST_F(LibYUVConvertTest, NV12Crop) {
   const int SUBSAMP_X = 2;
   const int SUBSAMP_Y = 2;
   const int kWidth = benchmark_width_;