test lib: change "error" to "BUG" as appropriate

Change two uses of "error" in test-lib-functions.sh to "BUG".

In the first instance in "test_cmp_rev" the author of the "BUG"
function added in [1] had another in-flight patch adding this in [2],
and the two were never consolidated.

In the second case in "test_atexit" added in [3] that we could have
instead used "BUG" appears to have been missed.

1. 165293a (tests: send "bug in the test script" errors to the
   script's stderr, 2018-11-19)

2. 30d0b6d (test-lib-functions: make 'test_cmp_rev' more
   informative on failure, 2018-11-19)

3. 900721e (test-lib: introduce 'test_atexit', 2019-03-13)

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

t/test-lib-functions.sh

@@ -1105,7 +1105,7 @@ test_cmp_rev () {
 	fi
 	if test $# != 2
 	then
-		error "bug in the test script: test_cmp_rev requires two revisions, but got $#"
+		BUG "test_cmp_rev requires two revisions, but got $#"
 	else
 		local r1 r2
 		r1=$(git rev-parse --verify "$1") &&
@@ -1216,7 +1216,7 @@ test_atexit () {
 	# doing so on Bash is better than nothing (the test will
 	# silently pass on other shells).
 	test "${BASH_SUBSHELL-0}" = 0 ||
-	error "bug in test script: test_atexit does nothing in a subshell"
+	BUG "test_atexit does nothing in a subshell"
 	test_atexit_cleanup="{ $*
 		} && (exit \"\$eval_ret\"); eval_ret=\$?; $test_atexit_cleanup"
 }