Suppress verbose vips logging output

Initialize libvips with LogLevelError to prevent info-level messages
from polluting the JSON log stream.

internal/imgcache/processor_test.go

@@ -15,7 +15,7 @@ import (
 )
 
 func TestMain(m *testing.M) {
-	vips.Startup(nil)
+	initVips()
 	code := m.Run()
 	vips.Shutdown()
 	os.Exit(code)