skip some skippable tests (#3102)

* tests: consolidate redundant, duplicated helper code

These functions currently exist in a sourced shell library, but there is
an identical copy of them in a single test file. Get rid of this
duplicate definition.

* tests: mark some tests as skipped if bc is not installed

`exit 77` is the GNU exitcode protocol for tests that cannot be run
because their prerequisites are not available. If `bc` is not installed,
and it often isn't, the test can be short-circuited instead of failing;
meson will report them in "warning yellow".

* tests: mark some tests as skipped if support is not compiled

Not all test cases can be cleanly marked as skipped -- sometimes,
multiple things are checked, and having one be unavailable should not
mean skipping all tests.

But in a couple cases, a test file only tests one thing, and that may be
skipped. In such cases, it can be semantically indicated in the test
harness report collection, that a skip occurred.

test/test_cli.sh

@@ -7,32 +7,6 @@
 
 . ./variables.sh
 
-# is a difference beyond a threshold? return 0 (meaning all ok) or 1 (meaning
-# error, or outside threshold)
-# 
-# use bc since bash does not support fp math
-break_threshold() {
-	diff=$1
-	threshold=$2
-	return $(echo "$diff <= $threshold" | bc -l)
-}
-
-# subtract, look for max difference less than a threshold
-test_difference() {
-	before=$1
-	after=$2
-	threshold=$3
-
-	$vips subtract $before $after $tmp/difference.v
-	$vips abs $tmp/difference.v $tmp/abs.v 
-	dif=$($vips max $tmp/abs.v)
-
-	if break_threshold $dif $threshold; then
-		echo "difference is $dif"
-		exit 1
-	fi
-}
-
 test_rotate() {
 	im=$1
 	inter=$2

test/test_connections.sh

@@ -12,4 +12,6 @@ if test_supported jpegload_source; then
 
 	# test max difference < 10
 	test_difference $image $tmp/x.png 10
+else
+	exit 77
 fi

test/test_stall.sh

@@ -14,4 +14,6 @@ if test_supported tiffload; then
 -1 -1 -1
 EOF
 	VIPS_STALL=1 $vips conv $tmp/x.tif $tmp/x2.tif $tmp/mask.con
+else
+	exit 77
 fi

test/variables.sh.in

@@ -32,9 +32,14 @@ test_supported() {
 
 # is a difference beyond a threshold? return 0 (meaning all ok) or 1 (meaning
 # error, or outside threshold)
+#
+# use bc since bash does not support fp math
 break_threshold() {
 	diff=$1
 	threshold=$2
+	if ! command -v bc >/dev/null; then
+		exit 77
+	fi
 	return $(echo "$diff <= $threshold" | bc -l)
 }