integration: add requires() function

This is similar to the testRequires() function in the Docker test suite.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

tests/integration/checkpoint.bats

@@ -12,9 +12,7 @@ function teardown() {
 }
 
 @test "checkpoint and restore" {
-  if [ ! -e "$CRIU" ] ; then
-    skip
-  fi
+  requires criu
 
   # criu does not work with external terminals so..
   # setting terminal and root:readonly: to false

tests/integration/helpers.bash

@@ -33,10 +33,34 @@ function runc() {
   run __runc "$@"
 }
 
+# Raw wrapper for runc.
 function __runc() {
   "$RUNC" --root "$ROOT" "$@"
 }
 
+# Fails the current test, providing the error given.
+function fail() {
+	echo "$@" >&2
+	exit 1
+}
+
+# Allows a test to specify what things it requires. If the environment can't
+# support it, the test is skipped with a message.
+function requires() {
+	for var in "$@"; do
+		case $var in
+			criu)
+				if [ ! -e "$CRIU" ]; then
+					skip "Test requires ${var}."
+				fi
+				;;
+			*)
+				fail "BUG: Invalid requires ${var}."
+				;;
+		esac
+	done
+}
+
 # Retry a command $1 times until it succeeds. Wait $2 seconds between retries.
 function retry() {
   local attempts=$1