tests: fix vtest being hindrance to error finding.

* tests/common.tl (vtest): Only if the expected expression
is :error or (quote :error) do we wrap the expansion and
evaluation of the test expression with exception handling,
because only then do we expect an error. When the test
expression is anything else, we don't intercept any errors,
and so problems in test cases are easier to debug now.

* tests/012/struct.tl: In one case we must initialize
the *gensym-counter* to 4 to compensate for the change
in vtest to get the same gensym numbers in the output.

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