exert the string-freeing function in a test

Author: nraynaud

test/test_invalid_geometry.jl

@@ -9,4 +9,5 @@
     polygon = LibGEOS._readgeom("POLYGON((0 0, 10 0, 10 10, 0 10, 0 0),
         (15 15, 15 20, 20 20, 20 15, 15 15))")
     @test !LibGEOS.isValid(polygon)
+    @test LibGEOS.GEOSisValidReason_r(LibGEOS._context.ptr, polygon) == "Hole lies outside shell[15 15]"
 end