ext,tests: Add 'very-long' tag for test length

This new tag will be used to specify tests to run Weekly (with the
'long' tests for Nightly, and the 'quick' tests for Kokoro/presubmit).

Change-Id: I088c2ef36cb14746e581eac5b1771f82250f2b98
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49326
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

ext/testlib/configuration.py

@@ -248,6 +248,7 @@ def define_constants(constants):
     constants.length_tag_type = 'length'
     constants.quick_tag = 'quick'
     constants.long_tag = 'long'
+    constants.very_long_tag = 'very-long'
 
     constants.host_isa_tag_type = 'host'
     constants.host_x86_64_tag = 'x86_64'
@@ -272,6 +273,7 @@ def define_constants(constants):
         constants.length_tag_type: (
             constants.quick_tag,
             constants.long_tag,
+            constants.very_long_tag,
         ),
         constants.host_isa_tag_type: (
             constants.host_x86_64_tag,