Fix compiler warnings in test_bitmapset

The macros doing conversions of/from "text" from/to Bitmapset were using
arbitrary casts with Datum, something that is not fine since
2a600a93c7.

These macros do not actually need casts with Datum, as they are given
already "text" and Bitmapset data in input.  They are updated to use
cstring_to_text() and text_to_cstring(), fixing the compiler warnings
reported by the buildfarm.  Note that appending a -m32 to gcc to trigger
32-bit builds was enough to reproduce the warnings here.

While on it, outer parenthesis are added to TEXT_TO_BITMAPSET(), and
inner parenthesis are removed from BITMAPSET_TO_TEXT(), to make these
macros more consistent with the style used in the tree, based on
suggestions by Tom Lane.

Oversights in commit 00c3d87a5c.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Author: Greg Burd <greg@burd.me>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/3027069.1758606227@sss.pgh.pa.us

src/test/modules/test_bitmapset/test_bitmapset.c

@@ -84,8 +84,8 @@ PG_FUNCTION_INFO_V1(test_random_operations);
 	} while (0)
 
 /* Encode/Decode to/from TEXT and Bitmapset */
-#define BITMAPSET_TO_TEXT(bms) (text *) CStringGetTextDatum(nodeToString((bms)))
-#define TEXT_TO_BITMAPSET(str) (Bitmapset *) stringToNode(TextDatumGetCString((Datum) (str)))
+#define BITMAPSET_TO_TEXT(bms) cstring_to_text(nodeToString(bms))
+#define TEXT_TO_BITMAPSET(str) ((Bitmapset *) stringToNode(text_to_cstring(str)))
 
 /*
  * Individual test functions for each bitmapset API function