test(tvix/eval): add identity tests for literal evaluation

Change-Id: Id3f37fbe341e15e9428ef1d579d61a514232c0e8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6132
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/tests/tvix_tests/README.md

@@ -0,0 +1,19 @@
+These tests are "native" to Tvix and exist in addition to the Nix test
+suite.
+
+All of these are straightforward code snippets which are expected to
+produce a certain result.
+
+# `identity-*` tests
+
+Files named `identity-*.nix` contain code that is supposed to produce
+itself exactly after evaluation.
+
+These are useful for testing literals.
+
+# `eval-okay-*` tests
+
+Files named `eval-okay-*.nix` contain code which is supposed to
+evaluate to the output in the corresponding `eval-okay-*.exp` file.
+
+This convention is taken from the original Nix test suite.