selftests: vm: add "test_hmm.sh" to TEST_FILES

The "test_hmm.sh" file used by run_vmtests.sh dose not be installed into
INSTALL_PATH.  Thus run_vmtests.sh can not call it in INSTALL_PATH:

  ---------------------------
  running ./test_hmm.sh smoke
  ---------------------------
  ./run_vmtests.sh: line 74: ./test_hmm.sh: No such file or directory
  [FAIL]
  -----------------------

Add "test_hmm.sh" to TEST_FILES so that it will be installed.

Link: https://lkml.kernel.org/r/20220521083825.319654-3-patrick.wang.shcn@gmail.comSigned-off-by: Patrick Wang <patrick.wang.shcn@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/vm/Makefile

@@ -92,6 +92,7 @@ endif
 TEST_PROGS := run_vmtests.sh
 
 TEST_FILES := test_vmalloc.sh
+TEST_FILES += test_hmm.sh
 
 KSFT_KHDR_INSTALL := 1
 include ../lib.mk