test/test-functions: use binaries from $BUILD_DIR or installed system

test/test-functions: use binaries from $BUILD_DIR or installed system

In Ubuntu CI, we test binaries from the installed system, not from
$BUILD_DIR, so use the appropriate binary.  Most of the calls to the
binaries are part of checking/processing asan-built binaries, and so
did not apply to Ubuntu CI, except for generating noise in the stderr
log like:

objdump: '/tmp/autopkgtest.83yGoI/build.fHB/src/test/TEST-01-BASIC/systemd-journald': No such file

However this also applies to the call to systemd-nspawn, which the debian
upstream test wrapper was sed-adjusting to use the installed binary
instead of the binary in $BUILD_DIR.  This commit allows removing that
sed processing of the test-functions file during Ubuntu CI test.