tests/qemu-iotests/testenv: Use the "r2d" machine for sh4/sh4eb

Commit 0ea0538fae516f9b4 removed the default machine of the sh4
binaries, so a lot of iotests are failing now without such a default
machine. Teach the iotest harness to use the "r2d" machine instead
to fix this problem.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

tests/qemu-iotests/testenv.py

@@ -244,6 +244,8 @@ def __init__(self, source_dir: str, build_dir: str,
             ('riscv32', 'virt'),
             ('riscv64', 'virt'),
             ('rx', 'gdbsim-r5f562n8'),
+            ('sh4', 'r2d'),
+            ('sh4eb', 'r2d'),
             ('tricore', 'tricore_testboard')
         )
         for suffix, machine in machine_map: