[PATCH 4/8] test-lib: allow an external program to supply TAP output

A program which produces its own TAP output can't use test-lib.sh,
because our TAP header and test counts conflict.

We used to support this with test_external, but dropped it in 5beca49a0b
(test-lib: simplify by removing test_external, 2022-07-28). That
interface let you mix shell tests with external TAP, which required
special handling for the test counts. But we can bring back a much
simpler variant: tell test-lib.sh up front that it will be a thin
wrapper for an external TAP generator, and let it stay out of the way.

Now you might ask what is the point of using test-lib.sh at all if it's
not going to generate any output. And the point is: it does other useful
setup, like handling logging, setting certain environment variables, and
so on. The next commit will wrap the clar unit-tests which will bring
some real-world benefits.

The only changes we need here are avoiding the TAP header and the plan
count. Note that if we skip the whole script via GIT_SKIP_TESTS, though,
we still have to emit a count (since we wouldn't run the external
command at all).

Signed-off-by: Jeff King <redacted>
---
 t/test-lib.sh | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 3d001b91da..3754db0ea7 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh

@@ -1277,8 +1277,11 @@ test_done () {
 
 		# Maybe print SKIP message
 		test -z "$skip_all" || skip_all="# SKIP $skip_all"
-		case "$test_count" in
-		0)
+		case "$test_external,$skip_all,$test_count" in
+		t,,*)
+			: nothing
+			;;
+		*,*,0)
 			say "1..$test_count${skip_all:+ $skip_all}"
 			;;
 		*)

@@ -1534,7 +1537,7 @@ then
 	BAIL_OUT 'You need to build test-tool; Run "make t/helper/test-tool" in the source (toplevel) directory'
 fi
 
-if test -n "$HARNESS_ACTIVE"
+if test -n "$HARNESS_ACTIVE" && test -z "$test_external"
 then
 	say "TAP version 13"
 	say "pragma +strict"

-- 
2.56.0.559.g7c1a1d8168