oeqa/selftest/recipetool: fix metadata corruption on meta layer

oeqa/selftest/recipetool: fix metadata corruption on meta layer

[YOCTO #15314]

test_recipetool_appendsrcfile_update_recipe_basic is using base-files as
test recipe but modifies it directly which can corrupt metadata for other
tests relying on this recipe.

So use mtd-utils-selftest as test recipe from meta-selftest to avoid
this kind of issues

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>