Expose tests/src/test_helpers

Make SSL test helper code potentially available to sample programs. This is
notably useful for `ssl_client2` and `ssl_server2`, which may reasonably use
some test helper code even in the default configuration.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

scripts/generate_visualc_files.pl

@@ -31,6 +31,7 @@ my @test_source_dirs = qw(
     framework/tests/src
     framework/tests/src/drivers
     tests/src
+    tests/src/test_helpers
 );
 my $test_header_dir = 'framework/tests/include/test';
 my $test_drivers_header_dir = 'framework/tests/include/test/drivers';