selftests: vm: install test_vmalloc.sh for run_vmtests

Add test_vmalloc.sh to TEST_FILES to make sure it gets installed for
run_vmtests.

Fixed below error:
./run_vmtests: line 217: ./test_vmalloc.sh: No such file or directory

Tested with: make TARGETS=vm install INSTALL_PATH=$PWD/x

Signed-off-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>