[Flang] Fix a test case that depends on stderr output of nullptr. (#73349)

This test case depends on the stderr output of a `nullptr` being
"(null)". However, it is empty string on AIX.
This PR is to initialize the `sourceFileName_` to be a specific string
to avoid that.

Author: DanielCChen

flang/unittests/Runtime/RuntimeCrashTest.cpp

@@ -43,9 +43,9 @@ TEST(TestTerminator, CheckFailedLocationTest) {
 }
 
 TEST(TestTerminator, CheckFailedTest) {
-  static Fortran::runtime::Terminator t;
+  static Fortran::runtime::Terminator t("someFileName");
   ASSERT_DEATH(t.CheckFailed("predicate"),
-      "RUNTIME_CHECK\\(predicate\\) failed at \\(null\\)\\(0\\)");
+      "RUNTIME_CHECK\\(predicate\\) failed at someFileName\\(0\\)");
 }
 
 //------------------------------------------------------------------------------