t/testenv.sh: fix macOS test temp directory name

In commit cc41ca5 we changed the
template used for our call to mktemp(1) to add a suffix of "XXXXXX"
in order to comply with the requirements of the GNU version of that
command, which expects at least 3 "X" characters in a template
for a temporary file or directory name.

However, on macOS, the BSD version of mktemp(1) treats these
as literal "X" characters if the -t option is used, because in
this case the following argument is interpreted as a prefix,
not a template.  Thus we actually create temporary directories
on macOS of the form /var/folders/.../git-lfs_TEMP.XXXXXX.<random>,
with 8 random characters appended by the command.

Since the "X" characters are unnecessary in this case, we adjust
our definition of the TEMPDIR_PREFIX variable to leave them off
when our tests are running on macOS.

t/testenv.sh

@@ -77,7 +77,12 @@ BINPATH="$ROOTDIR/bin"
 PATH="$BINPATH:$PATH"
 
 # Always provide a test dir outside our git repo if not specified
-TEMPDIR_PREFIX="git-lfs_TEMP.XXXXXX"
+if [ "$IS_MAC" -eq 1 ]; then
+  TEMPDIR_PREFIX="git-lfs_TEMP"
+else
+  TEMPDIR_PREFIX="git-lfs_TEMP.XXXXXX"
+fi
+
 if [ -z "$GIT_LFS_TEST_DIR" ]; then
     GIT_LFS_TEST_DIR=$(mktemp -d -t "$TEMPDIR_PREFIX")
     GIT_LFS_TEST_DIR=$(resolve_symlink $GIT_LFS_TEST_DIR)