test/test-functions: drop all prefixes

When parsing and installing binaries mentioned in Exec*= lines the
5ed0dcf4d5 commit added parsing logic to drop
prefixes, including handling duplicate exclamation marks. But this did not
handle arbitrary combination of multiple prefixes, ie. StartExec=+-/bin/sh was
parsed as -/bin/sh which then would fail to install.

Instead of using egrep and shell replacements, replace both with sed command
that does it all. This sed script extract a group of characters starting with a
/ up to the first space (if any) after the equals sign. This correctly handles
existing non-prefixed, prefixed, multiple-prefixed commands.

About half commands seem to repeat themself, thus sort -u cuts the list of
binaries to install about in half.

To validate change of behaviour both old and new functions were modified to
echo parsed binaries into separate files, and then diffed. The incorrect
-/bin/sh was missing in the new output.

Without this patch tests fail on default Ubuntu installs.

test/test-functions

@@ -504,9 +504,8 @@ install_execs() {
     export PKG_CONFIG_PATH=$BUILD_DIR/src/core/
     systemdsystemunitdir=$(pkg-config --variable=systemdsystemunitdir systemd)
     systemduserunitdir=$(pkg-config --variable=systemduserunitdir systemd)
-    egrep -ho '^Exec[^ ]*=[^ ]+' $initdir/{$systemdsystemunitdir,$systemduserunitdir}/*.service \
-         | while read i; do
-         i=${i##Exec*=}; i=${i##[@+\!-]}; i=${i##\!}
+    sed -n 's|^Exec[a-zA-Z]*=[^/]*\(/[^ ]*\).*|\1|gp' $initdir/{$systemdsystemunitdir,$systemduserunitdir}/*.service \
+         | sort -u | while read i; do
          # some {rc,halt}.local scripts and programs are okay to not exist, the rest should
          inst $i || [ "${i%.local}" != "$i" ] || [ "${i%systemd-update-done}" != "$i" ]
      done