feat: updating mf_getuniquefileref() to use native approach to get a unique fileref (which is 100 times faster than the old approach)

tests/crossplatform/mf_getuniquefileref.test.sas

@@ -0,0 +1,17 @@
+/**
+  @file
+  @brief Testing mf_getuniquefileref macro
+
+  <h4> SAS Macros </h4>
+  @li mf_getuniquefileref.sas
+  @li mp_assert.sas
+
+**/
+
+%mp_assert(
+  iftrue=(
+    "%substr(%mf_getuniquefileref(),1,1)"="#"
+  ),
+  desc=Checking for a temp fileref,
+  outds=work.test_results
+)