build: Fix testprocess invocation during "as-installed" tests

During build-time testing, we can use `$<TARGET_FILE:childprocess>` to
pass the just-built childprocess executable to it as an argument, but
during "as-installed" testing we need to use the installed path of
the childprocess executable instead.

Signed-off-by: Simon McVittie <smcv@debian.org>

test/template.test.in

@@ -1,3 +1,3 @@
 [Test]
 Type=session
-Exec=@installedtestsdir@/@exe@ @noninteractive_arguments@
+Exec=@installedtestsdir@/@exe@ @installed_arguments@