globset: add opt-in Arbitrary trait implementations

This feature is mandatory when using `Glob` in fuzz testing.

Closes #2720

fuzz/fuzz_targets/fuzz_glob.rs

@@ -0,0 +1,22 @@
+#![no_main]
+
+use std::str::FromStr;
+
+use globset::Glob;
+
+libfuzzer_sys::fuzz_target!(|glob_str: &str| {
+    let Ok(glob) = Glob::new(glob_str) else {
+        return;
+    };
+
+    let Ok(glob2) = Glob::from_str(glob_str) else {
+        return;
+    };
+
+    // Verify that a `Glob` constructed with `new` is the same as a `Glob`` constructed
+    // with `from_str`.
+    assert_eq!(glob, glob2);
+
+    // Verify that `Glob::glob` produces the same string as the original.
+    assert_eq!(glob.glob(), glob_str);
+});