tests: fix the memcheck test

tests: fix the memcheck test

The file was raising due to the undefined constant `verifiers`, leading
the test to be ignored.

./main.py list -v

would show this as:

```
Traceback (most recent call last):
  File "/gem5/tests/../ext/testlib/loader.py", line 239, in load_file
    execfile(path, newdict, newdict)
  File "/gem5/tests/gem5/memcheck/test_memcheck.py", line 32, in <module>
    verifiers=verifiers,
NameError: name 'verifiers' is not defined

Exception thrown while loading "/gem5/tests/gem5/memcheck/test_memcheck.py"
Ignoring all tests in this file.
```

The test has been modified to have an empty verifiers list, which should
still catch bugs since src/mem/mem_checker.cc has several asserts already
which would be caught by the test system due to the exit status.

Also make the following changes to the test:

-   move it to tests/gem5/memory/test.py with other memory tests.

    This makes it easier to see all the tests in the source in one go, and
    might make it easier to factor certain memory things out later on.

-   reduce --maxtick to a value that finishes in 7 seconds on a Lenovo
    ThinkPad P51.

    The previous value would take 50x longer, which feels too long for a
    single test.

-   enable --prefetchers to also stress a prefetcher in the test

Change-Id: I58bd598b1142f349d25fa4fa4a8e41529e61a9ee
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26807
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>