test(tvix/eval): add tests for internal formals dependencies

Formals can depend on each other when using another formal as a
default value.

This test ensures that the compiler's declaration and initialisation
order of formals is consistent with what actually happens in the VM.

Change-Id: Ibdabe262554e8066d67fac1ebc3b5a48ef626e18
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7948
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals-deferred.exp

@@ -0,0 +1 @@
+42

tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals-deferred.nix

@@ -0,0 +1,3 @@
+# Tests formals which have internal default values that must be deferred.
+
+({ optional ? defaultValue, defaultValue }: optional) { defaultValue = 42; }

tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals.exp

@@ -0,0 +1 @@
+42

tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals.nix

@@ -0,0 +1,3 @@
+# Tests formals which have internal default values.
+
+({ defaultValue, optional ? defaultValue }: optional) { defaultValue = 42; }