Clean up specs

This moves the specs to the directories suggested in
#1001, updates them to use get-function(), and documents
the exact issues that they're testing.

It also updates the output of one of them to be an error, which is
expected.

spec/core_function/meta/call/regression/arglist_is_valid/input.scss

@@ -0,0 +1,13 @@
+// Regression test for sass/libsass#2472
+//
+// Passing an argument list to `call()` was broken on LibSass. The call to
+// `dark()` throw an error complaining about the second argument being the wrong
+// type.
+
+@function dark($color, $args...) {
+  @return call(get-function('darken'), $color, $args...);
+}
+
+.test {
+  wrong-type: dark(#102030, 5%);
+}

spec/core_function/meta/call/regression/arglist_triggers_argument_error/error

@@ -0,0 +1,5 @@
+Error: Only one positional argument is allowed. All other arguments must be passed by name.
+  @return call(get-function('adjust-color'), $color, $args...);
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  /sass/spec/core_function/meta/call/regression/arglist_triggers_argument_error/input.scss 9:11   adjust()
+  /sass/spec/core_function/meta/call/regression/arglist_triggers_argument_error/input.scss 13:12  root stylesheet

spec/core_function/meta/call/regression/arglist_triggers_argument_error/error-ruby-sass

@@ -0,0 +1,3 @@
+Error: -5 is not a keyword argument for `adjust_color'
+        on line 9 of /sass/spec/core_function/meta/call/regression/arglist_triggers_argument_error/input.scss
+  Use --trace for backtrace.

spec/core_function/meta/call/regression/arglist_triggers_argument_error/input.scss

@@ -0,0 +1,14 @@
+// Regression test for sass/libsass#2472
+//
+// Passing an argument list to `call()` was broken on LibSass. The call to
+// `adjust()` should throw an error complaining about passing -5 by position
+// instead of by name, but in LibSass it threw an error complaining about "not
+// enough arguments".
+
+@function adjust($color, $args...) {
+  @return call(get-function('adjust-color'), $color, $args...);
+}
+
+.test {
+  too-few: adjust(#102030, -5);
+}

spec/core_function/meta/call/regression/arglist_triggers_argument_error/options.yml

@@ -0,0 +1,3 @@
+---
+:todo:
+- libsass # sass/libsass#2696

spec/core_function/meta/call/regression/arglist_triggers_argument_error/status

@@ -0,0 +1 @@
+65