Add specs for content-exists in control statements

See sass/libsass#2842

Author: xzyfer

spec/core_functions/content-exists/basic.hrx spec/core_functions/content-exists/basic/basic.hrx

@@ -0,0 +1,28 @@
+<===> input.scss
+@mixin test-content-exists() {
+  @if content-exists() {
+    @content;
+  }
+  @else {
+    content-exists: false;
+  }
+}
+
+.should-be-true {
+  @include test-content-exists();
+}
+
+.should-be-false {
+  @include test-content-exists() {
+    content-exists: true;
+  }
+}
+
+<===> output.css
+.should-be-true {
+  content-exists: true;
+}
+
+.should-be-false {
+  content-exists: false;
+}