fix(tc): only suspend booting after running plat tests

fix(tc): only suspend booting after running plat tests

1. When doing a normal boot, tc_bl31_common_platform_setup() should
simply configure the platform and return.

2. When we are running the platform tests instead,
tc_bl31_common_platform_setup() should run the tests then suspend
booting (and thus never return).

We were incorreclty suspending the boot in case 1 as well. Put that
code under a preprocessor condition (PLATFORM_TEST_NV_COUNTERS or
PLATFORM_TEST_TFM_TESTSUITE) to fix this.

Signed-off-by: Lauren Wehrmeister <lauren.wehrmeister@arm.com>
Change-Id: I7d20800e3bcd85261e2cdad325586d184e12a3e3