Add a convenience function to preserve a test's state.

- Add `Sq::preserve` to prevent the files created by an `Sq`
    instance from being deleted when the `Sq` instance is dropped.

  - Preserving the state makes it easier to debug a test that is
    failing.

tests/integration/common.rs

@@ -156,6 +156,10 @@ impl FileOrKeyHandle {
 
 pub struct Sq {
     base: TempDir,
+    // Whether to preserve the directory on exit.  Normally we clean
+    // it up, but preserving it can simplify debugging when a test
+    // fails.
+    preserve: bool,
     home: PathBuf,
     policy: PathBuf,
     certd: PathBuf,
@@ -163,6 +167,26 @@ pub struct Sq {
     scratch: AtomicUsize,
 }
 
+impl Drop for Sq {
+    fn drop(&mut self) {
+        if self.preserve {
+            self.preserve = false;
+
+            match TempDir::new() {
+                Ok(tmp) => {
+                    let base = std::mem::replace(&mut self.base, tmp);
+                    let path = base.into_path();
+                    eprintln!("Preserving state in {}", path.display());
+                }
+                Err(err) => {
+                    eprintln!("Error preserving state in {}: {}",
+                              self.base.path().display(), err);
+                }
+            }
+        }
+    }
+}
+
 impl Sq {
     /// Creates a new Sq context in a new, emphemeral home directory.
     /// The clock is set to the specified time.
@@ -181,6 +205,7 @@ impl Sq {
 
         Sq {
             base,
+            preserve: false,
             home,
             policy,
             certd,
@@ -200,6 +225,16 @@ impl Sq {
         Self::at(now)
     }
 
+    /// Preserves the files on exit.
+    ///
+    /// Normally we clean delete all files and directories.  This
+    /// suppresses that behavior.  Preserving the state can facilitate
+    /// debugging.  Normally, you'll only enable this to debug a test,
+    /// and then disable it again.
+    pub fn preserve(&mut self) {
+        self.preserve = true;
+    }
+
     /// Returns the base directory.
     ///
     /// The sequoia home directory is under the `home` subdirectory.