oeqa/selftest: wic: improve extra-partition plugin tests

oeqa/selftest: wic: improve extra-partition plugin tests

Increase the number of inputs to check several files can be copied. Change
the partition type to GPT in order to avoid logical partitions.

Test various file systems and check the wic image partitions match.

Simplify 'wic ls' commands (remove pipes) and use '-n' to specify wic-tools
sysroot and avoid host contamination.

Also dedent the code just after the wic image is built to close the .wks
temporary file when its goal is achieved.

Signed-off-by: Louis Rannou <louis.rannou@non.se.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>