test_runtime.sh: Change --rootfs from '' to '.'

From the spec [1]:

  path (string, required) Specifies the path to the root filesystem
  for the container. A directory MUST exist at the path declared by
  the field.

From POSIX [2]:

  The special filename dot shall refer to the directory specified by
  its predecessor.

Also from POSIX [3]:

  Dot

  In the context of naming files, the filename consisting of a single
  <period> character ( '.' ).

Also from POSIX [4]:

  Filename

  A sequence of bytes consisting of 1 to {NAME_MAX} bytes used to name
  a file... The filenames dot and dot-dot have special meaning.

Support for empty-string paths in the wild seems mixed.  Coreutils'
realpath complains:

  $ realpath ''
  realpath: ‘’: No such file or directory
  $ realpath --version
  realpath (GNU coreutils) 8.23
  ...
  $ lddtree $(command -v realpath)
  /usr/bin/realpath (interpreter => /lib64/ld-linux-x86-64.so.2)
      libc.so.6 => /lib64/libc.so.6
  $ equery belongs /lib64/libc.so.6
   * Searching for /lib64/libc.so.6 ...
  sys-libs/glibc-2.21-r1 (/lib64/libc-2.21.so)
  sys-libs/glibc-2.21-r1 (/lib64/libc.so.6 -> libc-2.21.so)

But Bash's cp seems to treat it as '.':

  $ cd /tmp/
  $ pwd
  /tmp
  $ cd ''
  $ pwd
  /tmp
  $ command -V cd
  cd is a shell builtin
  $ echo $BASH_VERSION
  4.3.42(1)-release

In the test script, the use of the empty string comes from c69f4f1
(Modify default root path to "rootfs", 2015-11-13).  The change was
likely because the default 'rootfs' isn't useful when runtimetest
needs to read the config.json from the root of the filesystem, so:

  ./config.json
  ./rootfs/runtimetest
  ./rootfs/...

doesn't work (because after the pivot runtimetest cannot see
config.json).  You could copy the config.json:

  ./config.json
  ./rootfs/config.json
  ./rootfs/runtimetest
  ./rootfs/...

but the current test test script just dispenses with the extra
'rootfs' directory:

  ./config.json
  ./runtimetest
  ./...

Why c69f4f1 switched to an empty string instead of '.' is unclear,
but I think we should prefer '.' for the POSIX-compliance reasons
outlined above.

[1]: https://github.com/opencontainers/runtime-spec/blob/v1.0.0-rc1/config.md#root-configuration
[2]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_12
[3]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_136
[4]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_170

Signed-off-by: W. Trevor King <wking@tremily.us>

test_runtime.sh

@@ -78,7 +78,7 @@ tar -xf  rootfs.tar.gz -C ${TESTDIR}
 cp runtimetest ${TESTDIR}
 
 pushd $TESTDIR > /dev/null
-ocitools generate "${TEST_ARGS[@]}" --rootfs ""
+ocitools generate "${TEST_ARGS[@]}" --rootfs '.'
 popd > /dev/null
 
 TESTCMD="${RUNTIME} start $(uuidgen)"