Refactor database cleanups to take the same path for #close and ruby's garbage collection.

FossilOrigin-Name: a2a8d99136cfa9e98e75cc262b8f9aefa5ed576d7af804416b8c8c7f4d4f4bdb

spec/mdbx/database_spec.rb

@@ -17,5 +17,25 @@ RSpec.describe( MDBX::Database ) do
 		expect( db.closed? ).to be_truthy
 	end
 
+	context 'an opened database' do
+
+		before( :each ) do
+			@db = described_class.open( TEST_DATABASE.to_s )
+		end
+
+		after( :each ) do
+			@db.close
+		end
+
+		it "fails if opened again within the same process" do
+			# This is a function of libmdbx internals, just testing
+			# here for behaviorals.
+			expect {
+				described_class.open( TEST_DATABASE.to_s )
+			}.
+			to raise_exception( MDBX::DatabaseError, /environment is already used/ )
+		end
+
+	end
 end