[Dexter] Allow retries on all dexter tests to avoid lldb-dap flakiness (#161847)

This isn't pretty but should help us keep the bot stable while issues
such as #158306 and #158311 are investigated

cross-project-tests/debuginfo-tests/dexter-tests/lit.local.cfg

@@ -1,2 +1,5 @@
 if "dexter" not in config.available_features:
     config.unsupported = True
+
+# LLDB through lldb-dap causes spurious failures.
+config.test_retry_attempts = 2

cross-project-tests/debuginfo-tests/dexter/feature_tests/lit.local.cfg

@@ -3,3 +3,6 @@ if "dexter" not in config.available_features:
 
 config.name = "DExTer feature tests"
 config.suffixes = [".cpp", ".c", ".test"]
+
+# LLDB through lldb-dap causes spurious failures.
+config.test_retry_attempts = 2