read the feature flags in sextest in the way sexc does

sextest resolves #+ and #- itself -- it reads the program and prints
what survives to sexc -- so a flag spelling it does not recognise
decides which branch gets compiled

tests/sex-programs/feature-flags.sex

@@ -1,6 +1,6 @@
-(compilation "-f alpha --features beta --features=gamma --no-platform-features")
+(compilation "-f alpha -f beta,gamma --features=delta --no-platform-features")
 (input)
-(output "alpha" "beta" "gamma" "elsewhere")
+(output "alpha" "beta" "gamma" "delta" "elsewhere")
 (return 0)
 
 ;;; The flags naming the features, in every spelling sexc takes. This
@@ -19,6 +19,7 @@
   #+alpha (puts "alpha")
   #+beta (puts "beta")
   #+gamma (puts "gamma")
+  #+delta (puts "delta")
   #-unix (puts "elsewhere")
   #+unix (puts "here")
   (return 0))