Add basic tests of lockfile generation

Signed-off-by: Stephen Sherratt <stephen@sherra.tt>

test/blackbox-tests/test-cases/pkg/lockfile-generation.t/mock-opam-repository/packages/bar/bar.0.0.1/opam

@@ -0,0 +1 @@
+opam-version: "2.0"

test/blackbox-tests/test-cases/pkg/lockfile-generation.t/mock-opam-repository/packages/bar/bar.0.4.0/opam

@@ -0,0 +1 @@
+opam-version: "2.0"

test/blackbox-tests/test-cases/pkg/lockfile-generation.t/mock-opam-repository/packages/bar/bar.0.5.0/opam

@@ -0,0 +1 @@
+opam-version: "2.0"

test/blackbox-tests/test-cases/pkg/lockfile-generation.t/mock-opam-repository/packages/baz/baz.0.0.1/opam

@@ -0,0 +1 @@
+opam-version: "2.0"

test/blackbox-tests/test-cases/pkg/lockfile-generation.t/mock-opam-repository/packages/baz/baz.0.1.0/opam

@@ -0,0 +1 @@
+opam-version: "2.0"

test/blackbox-tests/test-cases/pkg/lockfile-generation.t/mock-opam-repository/packages/foo/foo.0.0.1/opam

@@ -0,0 +1,5 @@
+opam-version: "2.0"
+depends: [
+    "baz" {>= "0.1"}
+    "bar" {>= "0.2"}
+]

test/blackbox-tests/test-cases/pkg/lockfile-generation.t/run.t

@@ -0,0 +1,85 @@
+Simple example of generating a lock file with Dune
+
+Generate a `dune-project` file.
+  $ cat >dune-project <<EOF
+  > (lang dune 3.8)
+  > (package
+  >  (name lockfile_generation_test)
+  >  (depends
+  >    foo
+  >    (bar (>= "0.3"))
+  >   ))
+  > EOF
+
+Run the solver and generate a lock directory.
+  $ dune pkg lock --opam-env=pure --opam-repository=mock-opam-repository
+  Selected the following packages:
+  bar.0.4.0
+  baz.0.1.0
+  foo.0.0.1
+  lockfile_generation_test.LOCAL
+
+Print the name and contents of each file in the lock directory separated by
+"---", sorting by filename for consistency.
+  $ find dune.lock -type f | sort | xargs -I{} sh -c "printf '{}:\n\n'; cat {}; printf '\n\n---\n\n'"
+  dune.lock/bar:
+  
+  (version 0.4.0)
+  (dev false)
+  
+  ---
+  
+  dune.lock/baz:
+  
+  (version 0.1.0)
+  (dev false)
+  
+  ---
+  
+  dune.lock/foo:
+  
+  (version 0.0.1)
+  (deps baz bar)
+  (dev false)
+  
+  ---
+  
+  dune.lock/lock.dune:
+  
+  (lang package 0.1)
+  
+  ---
+  
+  dune.lock/lockfile_generation_test:
+  
+  (version LOCAL)
+  (deps foo bar)
+  (dev true)
+  
+  ---
+  
+
+Regenerate the `dune-project` file introducing an unsatisfiable constraint.
+  $ cat >dune-project <<EOF
+  > (lang dune 3.8)
+  > (package
+  >  (name lockfile_generation_test)
+  >  (depends
+  >    foo
+  >    (bar (>= "0.6"))
+  >   ))
+  > EOF
+
+Run the solver again. This time it will fail.
+  $ dune pkg lock --opam-env=pure --opam-repository=mock-opam-repository
+  Error: Can't find all required versions.
+  Selected: baz.0.1.0 foo.0.0.1 lockfile_generation_test.LOCAL
+  - bar -> (problem)
+      foo 0.0.1 requires >= 0.2
+      lockfile_generation_test LOCAL requires >= 0.6
+      Rejected candidates:
+        bar.0.5.0: Incompatible with restriction: >= 0.6
+        bar.0.4.0: Incompatible with restriction: >= 0.6
+        bar.0.0.1: Incompatible with restriction: >= 0.2
+  [1]
+