mk: fix default rule of test subdirectory

When using "make -C app/test" (with RTE_SDK/RTE_TARGET adjusted)
without specifying the rule (all), the build is not done.
Indeed the default rule is not "all" anymore since there are some
rules added for external resources link.

It is fixed by adding a reference to "all" at the top of the file
which makes it the default rule.

Note that make app/test_sub (without environment variable) is preffered.

Fixes: ab64f5df8004 ("app/test: support resources externally linked")

Reported-by: Reshma Pattan <reshma.pattan@intel.com>
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Reshma Pattan <reshma.pattan@intel.com>