Implement testing of stage1 in ./validate --hadrian

While master doesn't support ./validate --hadrian yet, most of that patch is ready in !326 (closed) and will be split up in a few MRs to make review & landing easier. However, it doesn't yet implement testing of the stage 1 compiler, just stage 2. This ticket serves as a reminder to implement testing of stage 1 with hadrian in the validate script.

Trac metadata

Trac field	Value
Version	8.7
Type	Task
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Build System (Hadrian)
Test case
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changed weight to 7

added Ttask Trac import hadrian labels

added Phigh label

@alp With this recent commit (83a88988) to the validate script, the stage1 compiler is tested only when the "--fast" flag is used. Does this cover what you had in mind or should a specific flag be added for stage1 compiler?

@alinab I'm a bit fuzzy on the details after all that time, but I think it's fine if this mimics whatever happens with Make, so as to preserve the workflow of anyone used to testing things with validate (with Make).