Allow running test suite without libuhdr (#428)

Author: kleisauke

spec/image_spec.rb

@@ -727,7 +727,7 @@
   end
 
   # added in 8.18
-  if Vips.respond_to? :vips_image_get_gainmap
+  if has_uhdr?
     it "can modify gainmaps" do
       # test the example code in image.rb doc comment for get_gainmap
       def modify_gainmap image

spec/spec_helper.rb

@@ -23,11 +23,15 @@ def timg(name)
 end
 
 def has_jpeg?
-  Vips.type_find("VipsOperation", "jpegload") != nil
+  Vips.type_find("VipsOperation", "jpegload") != 0
 end
 
 def has_svg?
-  Vips.type_find("VipsOperation", "svgload") != nil
+  Vips.type_find("VipsOperation", "svgload") != 0
+end
+
+def has_uhdr?
+  Vips.type_find("VipsOperation", "uhdrload") != 0
 end
 
 RSpec.configure do |config|