fix: showing filerefs that exist (even when underlying does not) in mf_existfileref, along with 3 tests

tests/base/mf_existfileref.test.sas

@@ -0,0 +1,35 @@
+/**
+  @file
+  @brief Testing mf_existfileref macro
+
+  <h4> SAS Macros </h4>
+  @li mf_existfileref.sas
+  @li mp_assert.sas
+
+**/
+
+filename ref1 temp;
+filename ref2 temp;
+
+data _null_;
+  file ref1;
+  put 'exists';
+run;
+
+%mp_assert(
+  iftrue=(%mf_existfileref(ref1)=1),
+  desc=Checking fileref WITH target file exists,
+  outds=work.test_results
+)
+
+%mp_assert(
+  iftrue=(%mf_existfileref(ref2)=1),
+  desc=Checking fileref WITHOUT target file exists,
+  outds=work.test_results
+)
+
+%mp_assert(
+  iftrue=(%mf_existfileref(ref3)=0),
+  desc=Checking non-existant fref does not exist,
+  outds=work.test_results
+)