tests/postprocess: modify test_pkg_merge to check multi depends/provides

There was a mistake in parsing of Provides field when there was more
than one field. There was not test to check for this.
This commit modifies existing generic test to also check this case. It
just adds multi-dependency and multi-provide to first package.
9 jobs for missing-candidate in 16 minutes (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #347113
build-alpine

00:02:57

passed #347111
build-debian-stable

00:03:09

passed #347112
build-debian-unstable

00:02:57

 
  Test
passed #347118
cppcheck

00:02:46

passed #347119
luacheck

00:02:43

passed #347116
system-tests

00:11:31

passed #347117
system-valgrind-tests

00:12:51

passed #347114
unit-tests

00:07:41

passed #347115
unit-valgrind-tests

00:11:05