lang: funcs: core: test: Make a new instance for each test

Since this special one_instance function uses global state, if it's
re-used in more than one test, this won't work since they still all use
the whole global state. Make new ones for each test.

This also breaks the count=2 feature (any number other than 1) when
running these, which is not ideal. Create a cleanup API that we can run
between tests to reset the global state.

lang/interpret_test/TestAstFunc2/test-one-instance-lambda-arg.txtar

@@ -5,14 +5,14 @@ $double = func($x) {
 	$x + $x
 }
 
-# one_instance_a should only produce one value, and will error if initialized twice
+# one_instance_g should only produce one value, and will error if initialized twice
 test "test1" {
-	anotherstr => $double(test.one_instance_a()),
+	anotherstr => $double(test.one_instance_g()),
 }
 
-# one_instance_b should only produce one value, and will error if initialized twice
+# one_instance_h should only produce one value, and will error if initialized twice
 test "test2" {
-	anotherstr => $double(test.one_instance_b()),
+	anotherstr => $double(test.one_instance_h()),
 }
 -- OUTPUT --
 Vertex: test[test1]