Cygwin: testsuite: Drop Adminstrator privileges while running tests

Test access05 and symlink03 expect operations to fail which succeed when
we have Adminstrator privileges.

There's perhaps a bit of incoherency here: some XFAILed tests expect to
run as root (so maybe we need the ability to selectively cygdrop?).

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>

.github/workflows/cygwin.yml

@@ -73,6 +73,7 @@ jobs:
           automake,
           busybox,
           cocom,
+          cygutils-extra,
           dblatex,
           dejagnu,
           docbook-xml45,

winsup/doc/faq-programming.xml

@@ -697,8 +697,9 @@ Building these programs can be disabled with the <literal>--without-cross-bootst
 option to <literal>configure</literal>.
 </para>
 
-<!-- If you want to run the tests, <literal>dejagnu</literal> and
-     <literal>busybox</literal> are also required. -->
+<!-- If you want to run the tests, <literal>dejagnu</literal>,
+     <literal>busybox</literal> and <literal>cygutils-extra<literal> are also
+     required. -->
 
 <para>
 Building the documentation also requires the <literal>dblatex</literal>,

winsup/testsuite/winsup.api/winsup.exp

@@ -64,7 +64,7 @@ foreach src [lsort [glob -nocomplain $srcdir/$subdir/*.c $srcdir/$subdir/*/*.{cc
 	    }
 	    file mkdir $tmpdir/$tmpfile
 	    set env(PATH) "$runtime_root:$env(PATH)"
-	    ws_spawn "$cygrun $exec $testdll_tmpdir/$tmpfile > $redirect_output"
+	    ws_spawn "cygdrop $cygrun $exec $testdll_tmpdir/$tmpfile > $redirect_output"
 	    file delete -force $tmpdir/$tmpfile
 	    set env(PATH) "$orig_path"
 	    if { $rv } {