test: group runtests.jl into named testsets (#270)

Reorganize the monolithic test/runtests.jl into named @testset blocks
grouped by functional area (type conversion, DB open/close, tables
interface, execute/query, transactions, load!/drop!, strict mode, UDF
registration, serialization, Dates, backup, Stmt scope) under a single
top-level "SQLite" testset.

This is a pure reorganization with no semantic changes: no test is
added, removed, reordered across a state dependency, or weakened. The
assertion count is unchanged (155 @test + 22 @test_throws) and the suite
reports the same 181 passing tests as before. State-coupled blocks are
kept together and annotated; a couple of previously-shared tables are
recreated locally so each testset is self-contained.

Addresses #270.

Author: JohnCobbler