learning-gem5,tests: Move Learning gem5 Part 3 to run nightly

The Learning gem5 part 3 tests require the building of the X86_MSI
binary. These are the only tests that require this protocol. Building
this is not worth it to just run these tests. They've therefore been
moved to be run nightly rather than as a pre-submit/kokoro test.

Change-Id: If0cdd9c30a160a01cef5fcda8a5433ab2d6ac882
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50027
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

tests/gem5/learning_gem5/part3_test.py

@@ -43,6 +43,7 @@ gem5_verify_config(
     valid_isas=(constants.gcn3_x86_tag,),
     # dynamically linked
     valid_hosts=constants.target_host[constants.gcn3_x86_tag],
+    length=constants.long_tag,
 )
 
 gem5_verify_config(
@@ -53,4 +54,5 @@ gem5_verify_config(
     protocol = 'MSI',
     # Currently only x86 has the threads test
     valid_isas=(constants.gcn3_x86_tag,),
+    length=constants.long_tag,
 )