Use average mode for align raster downsample tests

GDAL 2.0 changed (fixed) the bilinear downsampling algorithm, so
switch to the average algorithm so that test results are the same
for GDAL versions >= 2.0 and < 2.0.

Author: nyalldawson

tests/src/analysis/testqgsalignraster.cpp

@@ -169,7 +169,7 @@ class TestAlignRaster : public QObject
       QgsAlignRaster align;
       QgsAlignRaster::List rasters;
       rasters << QgsAlignRaster::Item( SRC_FILE, tmpFile );
-      rasters[0].resampleMethod = QgsAlignRaster::RA_Bilinear;
+      rasters[0].resampleMethod = QgsAlignRaster::RA_Average;
       align.setRasters( rasters );
       align.setParametersFromRaster( SRC_FILE, QString(), QSizeF( 0.4, 0.4 ) );
       bool res = align.run();
@@ -190,7 +190,7 @@ class TestAlignRaster : public QObject
       QgsAlignRaster align;
       QgsAlignRaster::List rasters;
       rasters << QgsAlignRaster::Item( SRC_FILE, tmpFile );
-      rasters[0].resampleMethod = QgsAlignRaster::RA_Bilinear;
+      rasters[0].resampleMethod = QgsAlignRaster::RA_Average;
       rasters[0].rescaleValues = true;
       align.setRasters( rasters );
       align.setParametersFromRaster( SRC_FILE, QString(), QSizeF( 0.4, 0.4 ) );