suzerain: init relevant wall values in absence of baseflow

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suzerain: propagate flags to sub-tests

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suzerain: #2983 document zero_d2y case

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suzerain: #2983 document unconstrained_d2y case

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suzerain: #2983 document progress, remove stubs

Rips out test code that I'd used over the last couple of days
to attempt to get viscous subsonic outflow conditions working
on the nonreflecting upper boundary.

Tosses out a warning message on startup if the problem is
known to occur.  Hopefully this lets someone find the issue
down the line.

Updates the manual.nrbc/ticket2399.sh test case so people know
it is expected to fail until #2983 and #2957 are resolved.

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