oeqa: Add test for layer append and FILESPATH tracking

oeqa: Add test for layer append and FILESPATH tracking

This test actually tests a spectrum of bitbake functionality. Three
layers are created, one containing a recipe, one with a bbappend adding
a file to the recipe and another which overwrites the file in another
bbappend.

The correct outcomes in building the recipe are tested, with the file
in the final layer added, removed and then re-added.

This tests bitbake's cache handling as well as restoration from
sstate which happens in the final test phase.

Based on a test case from:
[YOCTO #7019]

(From OE-Core rev: e57437c0e02b148a878c8db91660674069fbd6e2)

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