tests: add edge cases of flakes

A new test, edge-cases, covers paths that work but have no scenario yet:
a flake given by an indirect reference, which the target fetches by its
locked reference; a remote flake in a subdirectory; a flake in a
subdirectory drawing inputs from sources; a dirty Git work tree; a flake
whose lock file needs updating; drawing inputs from a source that isn't
a directory; nested names of missing inputs; several sources sharing one
flake; and an input locked to a path that the target has.

The deploying machine gets a registry entry for the indirect reference.
Resolving it fails if the global registry can't be downloaded, which it
can't in the VM, and disabling the global registry with flake-registry
needs flakes, so the deploying machine enables them, like any deploying
machine using indirect references does.  The target doesn't, so checks
for flake support on targets still mean something.

1 files changed, 7 insertions, 5 deletions

diff --git a/README.md b/README.md
index 35788cd..e6a8981 100644
--- a/README.md
+++ b/README.md
@@ -576,11 +576,13 @@ flakes, as long as the inputs drawing from them are declared with
 
 ## Tests
 
-`tests/` contains NixOS VM tests for populate: one test per source type, and
-one for features spanning sources, such as flake inputs from sources.  Each
-test deploys every combination of a source type's options to the deploying
-machine itself, to a remote target as `root`, and to a remote target using
-`sudo`, and checks the result after deploying and after deploying again.
+`tests/` contains NixOS VM tests for populate: one test per source type, one
+for features spanning sources, such as flake inputs from sources, one for edge
+cases of flakes, such as unusual lock files, and one for the arguments
+`writeDeploy` passes to `nixos-rebuild`.  The tests deploy every combination
+of a source type's options to the deploying machine itself, to a remote target
+as `root`, and to a remote target using `sudo`, and check the result after
+deploying and after deploying again.
 The tests require KVM and nixpkgs with `pkgs.testers.runNixOSTest`
 (23.05 or later), and use `<nixpkgs>` by default.