tests,learning-gem5: Moved MIPS ISA test from part 1 to long

tests,learning-gem5: Moved MIPS ISA test from part 1 to long

The learning gem5 part 1 tests were the only "quick" tests requiring the
MIPS ISA to be compilated. This is a big cost for two very simple tests.
The MIPS ISA tests for learning gem5 part 1 have been moved to the
"long" tests.

Change-Id: I694541b4c7ea84e91262f29c67fb5ec2bbbc6fec
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26844
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>