Add runtime test suite for ema lib

[srid]

The ema package currently has no runtime test harness. ema-generics has a test-type-errors suite, but that only exercises compile-time behaviour for the generics derivations — there's no place to land hspec/tasty-style unit tests for ema itself.

This blocked adding a test for Ema.Dynamic.currentValue in #177, which would have needed:

• a new test-suite stanza in ema.cabal
• an hspec or tasty dep (neither currently pulled in)
• a test/Spec.hs + per-module spec layout

Doing that on its own PR would be ~50 lines of pure plumbing before any test logic lands. Better done once, as a standalone setup PR, after which future feature PRs can drop in new specs cheaply.

What a minimal setup looks like

test-suite test
  import:           extensions
  type:             exitcode-stdio-1.0
  hs-source-dirs:   test
  main-is:          Spec.hs
  build-depends:    base, ema, hspec
  default-language: Haskell2010

Then test/Ema/DynamicSpec.hs can exercise currentValue:

spec = describe "currentValue" $ do
  it "returns initial value before consumption" $ do
    (readNow, _) <- currentValue (Dynamic (42 :: Int, \_ -> pass))
    readNow `shouldReturn` 42
  it "tracks updates once the wrapped Dynamic is consumed" $ do
    (readNow, Dynamic (_, xf)) <- currentValue
      (Dynamic (0 :: Int, \send -> mapM_ send [1, 2, 3]))
    xf pass  -- drive the updater once
    readNow `shouldReturn` 3

Tracks the dependency chain: #177 wants these, downstream srid/emanote#649 relies on the behaviour in production.