t0061: fix with --with-dashes and RUNTIME_PREFIX

When building Git with RUNTIME_PREFIX and starting a test helper from
t/helper/, it fails to detect the system prefix correctly.

This is the reason that the warning

	RUNTIME_PREFIX requested, but prefix computation failed. [...]

to be printed.

In t0061, we did not expect that to happen, and it actually did not
happen in the normal case, because bin-wrappers/test-tool specifically
sets GIT_TEXTDOMAINDIR (and as a consequence, nothing in test-tool wants
to know about the runtime prefix).

However, with --with-dashes, bin-wrappers/test-tool is no longer called,
but t/helper/test-tool is called directly.

So let's just ignore the RUNTIME_PREFIX warning.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/t0061-run-command.sh

@@ -158,7 +158,8 @@ test_trace () {
 	expect="$1"
 	shift
 	GIT_TRACE=1 test-tool run-command "$@" run-command true 2>&1 >/dev/null | \
-		sed -e 's/.* run_command: //' -e '/trace: .*/d' >actual &&
+		sed -e 's/.* run_command: //' -e '/trace: .*/d' \
+			-e '/RUNTIME_PREFIX requested/d' >actual &&
 	echo "$expect true" >expect &&
 	test_cmp expect actual
 }