fish_indent: Add check

This isn't quite the old-style test, but it checks some of the line
continuation stuff.

Note that littlecheck ignores leading whitespace, so testing the
actual indentation requires some more effort.

tests/checks/indent.fish

@@ -0,0 +1,64 @@
+# RUN: %fish -C 'set -g fish_indent %fish_indent' %s
+# Test file for fish_indent
+
+echo 'echo foo \\
+| cat' | $fish_indent
+#CHECK: echo foo \
+#CHECK: | cat
+
+echo 'echo foo | \\
+cat' | $fish_indent
+#CHECK: echo foo | cat
+
+echo 'echo foo |
+cat' | $fish_indent
+#CHECK: echo foo |
+#CHECK: cat
+
+echo 'if true; \\
+    or false
+    echo something
+end' | $fish_indent
+#CHECK: if true; or false
+#CHECK: echo something
+#CHECK: end
+
+echo '\\
+echo wurst' | $fish_indent
+#CHECK: echo wurst
+
+echo 'echo foo \\
+brot' | $fish_indent
+#CHECK: echo foo \
+#CHECK: brot
+
+
+echo 'echo rabarber \\
+     banana' | $fish_indent
+#CHECK: echo rabarber \
+#CHECK: banana
+
+echo 'for x in a \\
+    b \\
+    c
+    echo thing
+end' | $fish_indent
+#CHECK: for x in a \
+#CHECK: b \
+#CHECK: c
+#CHECK: echo thing
+#CHECK: end
+    
+echo 'echo foo |
+echo banana' | $fish_indent
+#CHECK: echo foo |
+#CHECK: echo banana
+
+echo 'echo foo \\
+;' | $fish_indent
+#CHECK: echo foo \
+#CHECK: 
+
+echo 'echo foo \\
+' | $fish_indent
+#CHECK: echo foo \