chore: adding test for mf_existds

tests/crossplatform/mf_existds.test.sas

@@ -0,0 +1,25 @@
+/**
+  @file
+  @brief Testing mf_existfileref macro
+
+  <h4> SAS Macros </h4>
+  @li mf_existds.sas
+  @li mp_assert.sas
+
+**/
+
+data work.testme;
+x=1;
+run;
+
+%mp_assert(
+  iftrue=(%mf_existds(work.testme)=1),
+  desc=Checking existing dataset exists,
+  outds=work.test_results
+)
+
+%mp_assert(
+  iftrue=(%mf_existds(work.try2testme)=0),
+  desc=Checking non existing dataset does not exist,
+  outds=work.test_results
+)