unit-test: require meson 1.3.0 for relative_to calls

Meson can give the following warning:

```
WARNING: Project specifies a minimum meson_version '>=1.1.1' but uses features which were added in newer versions:
 * 1.3.0: {'fs.relative_to'}
```

Catch this in our CI.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I852950bfc7b02abae9d00fb5663136cfdc690736
diff --git a/scripts/unit-test.py b/scripts/unit-test.py
index d6d9d1b..eaab169 100755
--- a/scripts/unit-test.py
+++ b/scripts/unit-test.py
@@ -1182,7 +1182,15 @@
             ):
                 raise Exception(
                     "dep.get_variable() with positional argument requires "
-                    + "meson_Version: '>=0.58'"
+                    + "meson_version: '>=0.58'"
+                )
+
+        if "relative_to(" in build_contents:
+            if not meson_version or not meson_version_compare(
+                meson_version, ">=1.3.0"
+            ):
+                raise Exception(
+                    "fs.relative_to() requires meson_version: '>=1.3.0'"
                 )