Feature request: test sketches even when no platforms are selected

Currently, there is tension between the EXPECT_EXAMPLES configuration variable and the platform configuration for an example sketch. If EXPECT_EXAMPLES is set, it won't allow any individual example to be configured with no platforms (as might be the maintainers intention).

If no platforms are configured for a sketch, it would be better to simply perform a syntax check on the files themselves but not a full compilation. This would require backend support (not because I can't run gcc -c myself, but because I would be reliant on the backend to supply the correct list of files).

Depends on arduino/arduino-cli#1128