macaw-symbolic: Add more testing for SymbolicMutable option

[RyanGlScott]

The macaw-symbolic memory model has a somewhat obscure SymbolicMutable option:

macaw/symbolic/src/Data/Macaw/Symbolic/Memory.hs

Lines 166 to 169 in 97c61e4

	data MemoryModelContents = SymbolicMutable
	-- ^ All mutable global data is fully symbolic, but
	-- read-only data is concrete (and taken from the
	-- memory image of the binary)

Which is explained with:

macaw/symbolic/src/Data/Macaw/Symbolic/Memory.hs

Lines 158 to 162 in 97c61e4

	-- If we are trying to prove properties about functions in isolation, though, we
	-- have to assume that non-constant global data can have /any/ value, which
	-- means that we have to represent each byte as symbolic. This will change when
	-- we start pursuing compositional verification and want to have more elaborate
	-- memory setups.

I'm unclear how many downstream projects are actually using SymbolicMutable, but the macaw repo itself certainly isn't. The only use of it that I can find is in this bitrotted example (see also #324), which is not run on CI. Moreover, all of the code run in the macaw-symbolic test suites uses ConcreteMutable instead of SymbolicMutable, so SymbolicMutable is effectively not being tested right now.

At the very least, we should add a basic unit test of some kind to make sure that SymbolicMutable is actually doing what it claims to do.