selftests: Fix lib.mk run_tests target shell script

selftests: Fix lib.mk run_tests target shell script

Within run_tests target, the whole script needs to be executed within
the same shell and not as separate subshells, so the initial test_num
variable set to 0 is still present when executing "test_num=`echo
$$test_num+1 | bc`;".

Demonstration of the issue (make run_tests):

TAP version 13
(standard_in) 1: syntax error
selftests: basic_test
========================================
ok 1.. selftests: basic_test [PASS]
(standard_in) 1: syntax error
selftests: basic_percpu_ops_test
========================================
ok 1.. selftests: basic_percpu_ops_test [PASS]
(standard_in) 1: syntax error
selftests: param_test
========================================
ok 1.. selftests: param_test [PASS]

With fix applied:

TAP version 13
selftests: basic_test
========================================
ok 1..1 selftests: basic_test [PASS]
selftests: basic_percpu_ops_test
========================================
ok 1..2 selftests: basic_percpu_ops_test [PASS]
selftests: param_test
========================================
ok 1..3 selftests: param_test [PASS]

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Fixes: 1f87c7c15d7 ("selftests: lib.mk: change RUN_TESTS to print messages in TAP13 format")
CC: Shuah Khan <shuahkh@osg.samsung.com>
CC: linux-kselftest@vger.kernel.org
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>