Support start symbol directive in SPT

[molenzwiebel]

Summary

The start symbol directive in SPT is ignored, with the SPT tests opting to use the default start symbol instead.

What you did

language foo

start symbol Exp

test parse to num [[ 1 ]] parse to Int("1")

What you expected to happen

Test succeeds.

What actually happened

Test fails, since it is parsed from the start symbol (i.e. something like Program(Exprs([Int("1")]))).

Context

• Spoofax version: 0.11.9
• Operating system & version: all