selftest/recipetool: Stop test corrupting tinfoil class

selftest/recipetool: Stop test corrupting tinfoil class

Running recipetool.RecipetoolTests.test_recipetool_handle_license_vars
followed by wic.Wic2.test_biosplusefi_plugin_qemu would show a failure of:

  File "/media/build/poky/meta/lib/oeqa/utils/commands.py", line 351, in runqemu
    qemu = oeqa.targetcontrol.QemuTarget(recipedata, targetlogger, image_fstype)
  File "/media/build/poky/meta/lib/oeqa/targetcontrol.py", line 116, in __init__
    use_kvm = oe.types.qemu_use_kvm(d.getVar('QEMU_USE_KVM'), d.getVar('TARGET_ARCH'))
  File "/media/build/poky/meta/lib/oe/types.py", line 182, in qemu_use_kvm
    if kvm and boolean(kvm):
  File "/media/build/poky/meta/lib/oe/types.py", line 128, in boolean
    raise ValueError("Invalid boolean value '%s'" % value)
ValueError: Invalid boolean value '/media/build/poky/meta/files/common-licenses'

which made no sense until you realise the recipetool test is corrupting
the tinfoil class. Work on a copy instead to avoid this.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>