Enable globstar in run-test.sh

Globstar is not enabled by default, and a recent commit attempting to increase the number of directories being run for the Linux build actually skipped a bunch of stuff.  Enable globstar in the script (this is local) to fix.

Mark DriveLetter_Unix as a failing test, which appears to have snuck in in the meantime

Author: mmitche

run-test.sh

@@ -1,5 +1,8 @@
 #!/usr/bin/env bash
 
+# Enable globstar for the purposes of directory evaluation
+shopt -s globstar
+
 wait_on_pids()
 {
   # Wait on the last processes

src/System.IO.FileSystem/tests/Directory/CreateDirectory.cs

@@ -347,6 +347,7 @@ public void DriveLetter_Windows()
 
         [Fact]
         [PlatformSpecific(PlatformID.AnyUnix)]
+        [ActiveIssue(2459)]
         public void DriveLetter_Unix()
         {
             // On Unix, there's no special casing for drive letters, which are valid file names