deb: fix concurrent testing (by making them serial)

Co-authored-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>
Co-committed-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>

src/deb/mod.rs

@@ -105,6 +105,7 @@ fn find_dsc_file(
 
 #[cfg(test)]
 mod tests {
+    use serial_test::serial;
     async fn test_build_end_to_end(
         package: &str,
         series: &str,
@@ -159,8 +160,16 @@ mod tests {
         );
     }
 
+    // Tests below will be marked 'serial'
+    // As builds are using ephemeral contexts, tests running on the same
+    // process could use the ephemeral context of another thread and
+    // interfere with each other.
+    // FIXME: This is not ideal. In the future, we might want to
+    // either explicitely pass context (instead of shared state) or
+    // fork for building?
     #[tokio::test]
     #[test_log::test]
+    #[serial]
     async fn test_deb_hello_ubuntu_end_to_end() {
         test_build_end_to_end("hello", "noble", None, None, false).await;
     }
@@ -168,6 +177,7 @@ mod tests {
     #[tokio::test]
     #[test_log::test]
     #[cfg(target_arch = "x86_64")]
+    #[serial]
     async fn test_deb_hello_ubuntu_cross_end_to_end() {
         test_build_end_to_end("hello", "noble", None, Some("riscv64"), true).await;
     }