scripts/oe-selftest: Add filtering to the coverage data gathered by oe-selftest

scripts/oe-selftest: Add filtering to the coverage data gathered by oe-selftest

When --coverage is used, oe-selftest gathers coverage data from the testcases
executed. The command lacks a way of filtering which files to gather coverage
data from.

This patch adds three options to specify which files should be considered.
The --coverage-source option specifies folders, while --coverage-include and
--coverage-omit specify patterns to have an extra level of filtering.

Some examples:

1. oe-selftest --run-all-tests --coverage

Gathers coverage data from the default poky folders

2. oe-selftest --run-all-tests --coverage --coverage-include /home/me/poky/scripts/*

Gathers coverage data only for the files located under '/home/me/poky/scripts'

3. oe-selftest --run-all-tests -coverage --coverage-omit /home/me/poky/meta*

Gathers coverage data. Files inside all the folders starting with 'meta' under
'/home/me/poky' are omited

4. oe-selftest --run-all-tests --coverage --coverage-source /home/me/poky/bitbake

Gathers coverage data only from files inside the folder: '/home/me/poky/bitbake'

[Yocto #8920]

Signed-off-by: Humberto Ibarra <humberto.ibarra.lopez@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>