Execute tests in independent testing roots to enable parallel running

Up to now we've run tests in a mish-mash of an environment where bits
and pieces of previous tests might exist, and as state has been shared
it's been impossible to execute tests in parallel.

This makes the pre-populated testing-root read-only to the owner to force
all writers to perform an extra setup call to gain a private testing
environment. Most of the users needing this already had RPMDB_INIT calls
to ensure clean rpmdb state so that's where this is hooked onto.

There's a fair bit of gymnastics with the environment to make things match
on both sides of fakechroot, some of which can hopefully go away eventually
once the dust from this settles. This is also rather heavy as it is,
on my laptop serial execution goes down from ~1m15s to ~1m45s, but then
parallel execution with -j8 is down to ~50s. There should be a number of
optimizations that can be made, such as setting up links for writable
directories instead of copying the entire testing-tree around, but leaving
that as a future exercise. This is more of an enabler than the goal state.

Note that this does *not* enable test-suite parallel running for
`make -jN check`, because `make` does not export the -j argument in
a way that we could pass to `./rpmtests` from the makefile. To enable
that, one needs to pass a suitable -jN argument through TESTSUITEFLAGS,
such as `make -j8 check TESTSUITEFLAGS="-j8"`. Or run invoke `rpmtests`
manually.

tests/debugedit.at

@@ -26,6 +26,7 @@ m4_define([RPM_DEBUGEDIT_SETUP],[[
 # Create some test binaries. Create and build them in different subdirs
 # to make sure they produce different relative/absolute paths.
 
+export HOME=${PWD}
 mkdir subdir_foo
 cp "${abs_srcdir}"/data/SOURCES/foo.c subdir_foo
 mkdir subdir_bar