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

@@ -0,0 +1,24 @@
+(compilation "-f alpha --features beta --features=gamma --no-platform-features")
+(input)
+(output "alpha" "beta" "gamma" "elsewhere")
+(return 0)
+
+;;; The flags naming the features, in every spelling sexc takes. This
+;;; is not pedantry about the command line: sextest reads the program
+;;; itself and prints the surviving forms to sexc, so a spelling it
+;;; does not recognise leaves the guards below resolved against the
+;;; wrong set -- quietly, since the test then checks the output of a
+;;; program it did not mean to compile.
+;;;
+;;; --no-platform-features is what makes `#-unix' true wherever this is
+;;; compiled, and it has to be honoured on both sides for that to hold.
+
+(include stdio.h)
+
+(pub fn main () int
+  #+alpha (puts "alpha")
+  #+beta (puts "beta")
+  #+gamma (puts "gamma")
+  #-unix (puts "elsewhere")
+  #+unix (puts "here")
+  (return 0))