Disable test on xplat, since failfast is always treated as test failure there

test/Regex/Bug15992535-xplat.baseline

@@ -1 +0,0 @@
-

test/Regex/rlexe.xml

@@ -221,19 +221,15 @@
   <test>
     <default>
       <files>Bug15992535.js</files>
-      <!-- Test should fail assertion in debug build, so do not run. -->
+      <!-- Test should fail assertion in debug build, so do not run.  Also,
+           our test harness interprets a failfast as test failure on Linux
+           and MacOS (presumably because of a non-zero exit code from ch)
+           but not on Windows, so skip this test on xplat.
+
+           (The failfast here is temporary behavior; see GitHub issue #4722.)
+      -->
       <tags>exclude_chk,exclude_xplat</tags>
       <baseline>Bug15992535.baseline</baseline>
     </default>
   </test>
-  <!-- Mac/Linux variant of above test, which expects the additional
-       newline generated by the fail-fast on those platforms. -->
-  <test>
-    <default>
-      <files>Bug15992535.js</files>
-      <!-- Test should fail assertion in debug build, so do not run. -->
-      <tags>exclude_chk,exclude_win7,exclude_win8,exclude_winBlue,exclude_win10</tags>
-      <baseline>Bug15992535-xplat.baseline</baseline>
-    </default>
-  </test>
 </regress-exe>