Use unknowns(lv) to derive Tables interface names in symbol_indexing test

MTK 11 reorders ODESystem unknowns ([y, x] in this case, not [x, y]), so
the hardcoded propertynames in the test/downstream/symbol_indexing.jl
Tables interface check no longer match what `propertynames(row)` and
`Tables.columnnames(row)` return on the solution rows. The data columns
in `Array(sol_ts)'` already follow MTK's unknowns ordering, so deriving
the names list from `unknowns(lv)` keeps names and data in sync and is
robust to future MTK reorderings.

Restores 357/358 assertions in the `DiffEqArray Indexing Tests` testset
on Julia 1/lts/pre. The remaining 1-test error is in the array-symbolic
indexing test at line 103 (`sol[x .+ [y, 2y, 3y]] ≈ ...`) — passes
standalone, errors only inside `@safetestset`'s isolated module; this
looks like a separate Symbolics/SII module-isolation interaction that
needs its own investigation.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>

Author: ChrisRackauckas

test/downstream/symbol_indexing.jl

@@ -77,7 +77,7 @@ ts = 0:0.5:10
 sol_ts = sol(ts)
 @assert sol_ts isa DiffEqArray
 test_tables_interface(
-    sol_ts, [:timestamp, Symbol("x(t)"), Symbol("y(t)")],
+    sol_ts, [:timestamp; Symbol.(string.(unknowns(lv)))],
     hcat(ts, Array(sol_ts)')
 )