test: add helper macro ATX_RUN

This is meant to be a workaround for a limitation of Autotest,
where `set -e` has no effect in AT_CHECK().

Also we don't want to use `trap ... ERR`, since it is not portable.

tests/lib_sandbox.at

@@ -18,8 +18,10 @@ ATX_CHECK_LIB([
     chmod +x $tmpdir/false.sh
 
     # Check both read-only and read/write mode.
-    check-util-lib sandbox -r -d $tmpdir
-    check-util-lib sandbox    -d $tmpdir
+    ATX_RUN([
+        check-util-lib sandbox -r -d $tmpdir
+        check-util-lib sandbox    -d $tmpdir
+    ])
 ])
 
 AT_CLEANUP

tests/lib_watcher.at

@@ -14,7 +14,10 @@ ATX_CHECK_LIB([
     tmpdir=./$(ath_fn_rand_u64_hex).tmp.d
 
     mkdir $tmpdir
-    check-util-lib watcher -d $tmpdir
+
+    ATX_RUN([
+        check-util-lib watcher -d $tmpdir
+    ])
 ])
 
 AT_CLEANUP

tests/testsuite.at

@@ -46,12 +46,34 @@ m4_define([ATX_FEAT_PREREQ], [
     fi
 ])
 
+dnl
+dnl  ATX_RUN_ONE(cmd)
+dnl
+dnl  Run a pipeline of shell command(s).
+dnl  If it exits with a non-zero status, fail the current test.
+dnl
+m4_define([ATX_RUN_ONE], [
+    m4_ifnblank([$1], [
+        if $1; then :
+        else
+            echo "exit status: $?"
+            ATX_FAIL
+        fi
+    ])
+])
+
+dnl
+dnl  ATX_RUN(cmd_list)
+dnl
+dnl  Like ATX_RUN_ONE, but takes multiple pipelines, separated by newline.
+dnl  If any exits with a non-zero status, fail the current test.
+dnl
+m4_define([ATX_RUN], [
+    m4_map([ATX_RUN_ONE], m4_split([$1], m4_newline([+])))
+])
+
 m4_define([ATX_CHECK_SIMPLE], [
-    AT_CHECK([
-        atx_line_start=$LINENO
-        trap 'echo "Error $? at line $(($LINENO-$atx_line_start))"; exit 1' ERR
-        $1
-    ], , [ignore], [ignore])
+    AT_CHECK([$1], , [ignore], [ignore])
 ])
 
 m4_define([ATX_CHECK_LIB], [