run.py: allow python lambda expression for requires

Add a new require type, "lambda" which will require the Python lambda
expression to return a truthy value.

README.md

@@ -72,6 +72,10 @@ requires:
     - command2
     - ...
 
+  # Require the output of a Python expression to be true. For example,
+  # this will run on all platforms other than win32.
+  lambda: "sys.platform != win32"
+
 skip:
   # Skip a test if a feature is present, with a message that is logged.
   - feature: RUST

run.py

@@ -365,6 +365,9 @@ def check_requires(requires, suricata_config: SuricataConfig):
         elif key == "pcap":
             # A valid requires argument, but not verified here.
             pass
+        elif key == "lambda":
+            if not eval(requires["lambda"]):
+                raise UnsatisfiedRequirementError(requires["lambda"])
         else:
             raise Exception("unknown requires types: %s" % (key))