[lldb][test] Skip TestRerunAndExprDylib on Ubuntu 18.04

[kastiglione]

Summary

Disable this test on Ubuntu 18.04, where it fails for yet to be determined reasons.

[Michael137]

seems reasonable

lldb/test/API/functionalities/rerun_and_expr_dylib/TestRerunAndExprDylib.py

 """
 Test that re-running a process from within the same target
 after rebuilding the a dynamic library flushes the scratch
 TypeSystems tied to that process.
 """
 
 import lldb
 from lldbsuite.test.lldbtest import *
 from lldbsuite.test import lldbutil
 from lldbsuite.test.decorators import *
 
+
+def isUbuntu18_04():
+    """
+    Check if the host OS is Ubuntu 18.04.
+    Derived from `platform.freedesktop_os_release` in Python 3.10.
+    """
+    for path in ("/etc/os-release", "/usr/lib/os-release"):
+        if os.path.exists(path):
+            with open(path) as f:
+                contents = f.read()
+            if "Ubuntu 18.04" in contents:
+                return True
+
+    return False
+
+
 class TestRerunExprDylib(TestBase):
+    @skipTestIfFn(isUbuntu18_04, bugnumber="rdar://103831050")
     @skipIfWindows
     def test(self):
         """
         Tests whether re-launching a process without destroying
         the owning target keeps invalid ASTContexts in the
         scratch AST's importer.
 
         We test this by: