[librsvg: 1/2] Remove the need to set OUT_DIR in order to run 'cargo test'

[Federico Mena Quintero <federico src gnome org>]

commit 5b653a5a72cb667087e7afe82bb2f5a4fc7ea262
Author: Sven Neumann <sven svenfoo org>
Date:   Sun Oct 25 15:43:46 2020 +0100

    Remove the need to set OUT_DIR in order to run 'cargo test'
    
    If OUT_DIR is not set, a folder in the temporary directory as
    determined by std::env::temp_dir() will be used.

 rsvg_internals/src/test_utils.rs | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)
---
diff --git a/rsvg_internals/src/test_utils.rs b/rsvg_internals/src/test_utils.rs
index 065ace1b..5b071eda 100644
--- a/rsvg_internals/src/test_utils.rs
+++ b/rsvg_internals/src/test_utils.rs
@@ -45,17 +45,22 @@ impl Deviation for Diff {
     }
 }
 
-/// Creates a directory from the `OUT_DIR` environment variable and returns its path.
+/// Creates a directory for test output and returns its path.
+///
+/// The location for the output directory is taken from the `OUT_DIR` environment
+/// variable if that is set. Otherwise std::env::temp_dir() will be used, which is
+/// a platform dependent location for temporary files.
 ///
 /// # Panics
 ///
-/// Will panic if the `OUT_DIR` environment variable is not set.  Normally this is set
-/// by the continuous integration scripts or the build scripts that run the test suite.
+/// Will panic if the output directory can not be created.
 pub fn output_dir() -> PathBuf {
-    let path = PathBuf::from(
-        env::var_os("OUT_DIR")
-            .expect(r#"OUT_DIR is not set, please set it to a directory where the test suite can write its 
output"#),
-    );
+    let tempdir = || {
+        let mut path = env::temp_dir();
+        path.push("rsvg-test-output");
+        path
+    };
+    let path = env::var_os("OUT_DIR").map_or_else(tempdir, PathBuf::from);
 
     fs::create_dir_all(&path).expect("could not create output directory for tests");