[Flang] Testcase changes to switch directories before rm (NFC) (llvm#73602)

In AIX, the following testcase fails in attempt to delete the current
directory, exiting current directory before removing

---------

Co-authored-by: Mark Danial <[email protected]>

Author: madanial0

flang/test/Driver/write-module.f90

@@ -9,27 +9,31 @@
 ! RUN: rm -rf %t && mkdir -p %t/dir-flang
 ! RUN: cd %t && %flang -fsyntax-only -module-dir %t/dir-flang %s
 ! RUN: ls %t/dir-flang/testmodule.mod && not ls %t/testmodule.mod
+! RUN: cd -
 
 !--------------------------
 ! -module-dir<value>
 !--------------------------
 ! RUN: rm -rf %t && mkdir -p %t/dir-flang
 ! RUN: cd %t && %flang -fsyntax-only -module-dir%t/dir-flang %s
 ! RUN: ls %t/dir-flang/testmodule.mod && not ls %t/testmodule.mod
+! RUN: cd -
 
 !---------------------------
 ! -J <value>
 !---------------------------
 ! RUN: rm -rf %t && mkdir -p %t/dir-flang
 ! RUN: cd %t && %flang -fsyntax-only -J %t/dir-flang %s
 ! RUN: ls %t/dir-flang/testmodule.mod && not ls %t/testmodule.mod
+! RUN: cd -
 
 !------------------------------
 ! -J<value>
 !------------------------------
 ! RUN: rm -rf %t && mkdir -p %t/dir-flang
 ! RUN: cd %t && %flang -fsyntax-only -J%t/dir-flang %s
 ! RUN: ls %t/dir-flang/testmodule.mod && not ls %t/testmodule.mod
+! RUN: cd -
 
 module testmodule
   type::t2