support/testing: test_xen: rename TestXen to TestXenAarch64

Commit [1] introduced an Armv7 test case for Xen named TestXenArmv7,
next to the original Aarch64 test case which was simply named TestXen.

The test list shows, for example with the command
"support/testing/run-tests -l":

    tests.package.test_xen.TestXen
    tests.package.test_xen.TestXenArmv7

In order to make this test list a bit more explicit, this commit
renames the TestXen to TestXenAarch64. With that change, the list
becomes:

    tests.package.test_xen.TestXenAarch64
    tests.package.test_xen.TestXenArmv7

[1] https://gitlab.com/buildroot.org/buildroot/-/commit/5346824a83488f840008eb24f84866435ac32a12

Cc: Vincent Stehlé <vincent.stehle@arm.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

support/testing/tests/package/test_xen.py

@@ -89,7 +89,7 @@ class TestXenBase(infra.basetest.BRTest):
         self.assertNumVM(2)
 
 
-class TestXen(TestXenBase):
+class TestXenAarch64(TestXenBase):
     # Test Xen on 64b Arm.
     # Boot flow: Qemu Devicetree -> U-Boot -> Xen UEFI -> Linux
     # We need to boot Xen in UEFI to read xen.cfg.