test-lib: verbose mode for only tests matching a pattern

With the new --verbose-only=<pattern> option, one can enable --verbose
at a per-test granularity.  The pattern is matched against the test
number, e.g.

  ./t0000-basic.sh --verbose-only='2[0-2]'

to see only the full output of test 20-22, while showing the rest in the
one-liner format.

As suggested by Jeff King, this takes care to wrap the entire
test_expect_* block, but nothing else, in the verbose toggling.  We
can use the test_start/end functions from the previous commit for the
purpose.

This is arguably not *too* useful on its own, but makes the next patch
easier to follow.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0000-basic.sh

@@ -250,6 +250,30 @@ test_expect_success 'test --verbose' '
 	EOF
 '
 
+test_expect_success 'test --verbose-only' '
+	test_must_fail run_sub_test_lib_test \
+		test-verbose-only-2 "test verbose-only=2" \
+		--verbose-only=2 <<-\EOF &&
+	test_expect_success "passing test" true
+	test_expect_success "test with output" "echo foo"
+	test_expect_success "failing test" false
+	test_done
+	EOF
+	check_sub_test_lib_test test-verbose-only-2 <<-\EOF
+	> ok 1 - passing test
+	> Z
+	> expecting success: echo foo
+	> foo
+	> Z
+	> ok 2 - test with output
+	> Z
+	> not ok 3 - failing test
+	> #	false
+	> # failed 1 among 3 test(s)
+	> 1..3
+	EOF
+'
+
 test_set_prereq HAVEIT
 haveit=no
 test_expect_success HAVEIT 'test runs if prerequisite is satisfied' '