selftest/wic: code cleanup

selftest/wic: code cleanup

The following changes were added to improve the code readability:

- Removed redundant backlashes between brackets
- Aligned continuation lines according to PEP8 style
- Refactored command execution in the next methods for enhanced
  legibility:

  - test_build_image_name(self)
  - test_gpt_image(self)
  - test_qemux86_directdisk(self)
  - test_rootfs_indirect_recipes(self)
  - test_iso_image(self)
  - test_mkgummidisk(self)
  - test_mkefidisk(self)
  - test_directdisk_bootloader_config(self)
  - test_qemu(self)
  - test_bmap(self)
  - test_systemd_bootdisk(self)
  - test_sdimage_bootpart(self)

(From OE-Core rev: 5ba21836fe3c061a6271f855a9e26c66004d330a)

Signed-off-by: Jair Gonzalez <jair.de.jesus.gonzalez.plascencia@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>