[test] Ensure that all TLS structure descriptor mappings are valid

Constructing exhaustive standalone tests for all TLS structures is
time-consuming and of limited value, since there would be no guarantee
that the construction logic used in the test case matched the
construction logic matched in the real TLS protocol engine.

We already exercise each failure path that can be triggered by genuine
runtime errors (e.g. malformed received data, or transmit data that is
too large to fit within the relevant length field).  This leaves
untested the other major class of errors: an invalid structure
descriptor mapping (e.g. a missing field description) that would
render the structure impossible to parse or build.

Exercise the parser and builder for every structure type by creating
an all-zero descriptor (which is always permitted), sizing and
building the empty structure, and then parsing that structure.  This
ensures that the parser and builder are both satisfied that the
descriptor mapping is valid, since the validity of the mapping is
independent of the data being built or parsed.

Signed-off-by: Michael Brown <mcb30@ipxe.org>