Bump up Phantom TEST instance Size

Splunk support bugs me about the TEST instance size every time i contact them.

test/aws-us-gov/mdr-test-c2/account.hcl

@@ -160,7 +160,7 @@ locals {
     "nessus_security_center" = "m5a.xlarge",
     "nessus_scanners" = "m5a.large",
     "nessus_managers" = "m5a.large",
-    "phantom"        = "t3a.medium", # legacy test: t2.medium, legacy prod: m4.4xlarge
+    "phantom"        = "t3a.xlarge", # legacy test: t2.medium, legacy prod: m4.4xlarge
     "qcompliance"    = "t3a.small", # legacy: ? not sure
     "rhsso"          = "t3a.large",
     "rhsso-db"       = "db.t3.micro"