Add testing_check_is_nightly() function (#1290)

* Add testing_check_is_nightly() function

Will be required when turning off long-running tests.

* Update src/sst/core/testingframework/sst_unittest_support.py
---------

Co-authored-by: Eric Berquist <727571+berquist@users.noreply.github.com>

Author: jmlapre

src/sst/core/testingframework/sst_unittest_support.py

@@ -75,6 +75,15 @@ def __str__(self) -> str:
 # Commandline Information Functions
 ################################################################################
 
+def testing_check_is_nightly() -> bool:
+    """If Nightly is in the BUILD_TAG it's very likely a Nightly build."""
+    build_tag = os.getenv("BUILD_TAG")
+    if build_tag is not None and "Nightly" in build_tag:
+        return True
+    return False
+
+###
+
 def testing_check_is_in_debug_mode() -> bool:
     """ Identify if test frameworks is in debug mode