'main' tests: Don't assume ps(1) is available.

It's actually unavailable in the minimal chroots Debian builds our
package on.  That's allowed by POSIX, which specifies ps(1) to be
optional, whereas id(1) —

- is not optional in POSIX
- should exist on every system anyone might run the testsuite on
- has the same length name, so test expectations don't have to be updated
- doesn't take a filename argument (ditto)

That does make the pipeline as a whole somewhat nonsensical
semantically, but it remains just as valid syntactically.

highlighters/main/test-data/multiple-redirections.zsh

@@ -27,7 +27,7 @@
 # vim: ft=zsh sw=2 ts=2 et
 # -------------------------------------------------------------------------------------------------
 
-BUFFER='ps aux | grep java | sort | uniq | tail | head'
+BUFFER='id bob | grep java | sort | uniq | tail | head'
 
 expected_region_highlight=(
   "1  2  command" # ps

highlighters/main/test-data/simple-redirection.zsh

@@ -27,7 +27,7 @@
 # vim: ft=zsh sw=2 ts=2 et
 # -------------------------------------------------------------------------------------------------
 
-BUFFER='ps aux | grep java'
+BUFFER='id bob | grep java'
 
 expected_region_highlight=(
   "1  2  command" # ps