Added SHA256 test certificates

With SHA-1 deprecation, we need a few certificates using algorithms in
the default support list. Most tests still use SHA-1 though.

The generation process for the new certificates is recorded in the makefile.
diff --git a/tests/data_files/print_c.pl b/tests/data_files/print_c.pl
new file mode 100755
index 0000000..b584578
--- /dev/null
+++ b/tests/data_files/print_c.pl
@@ -0,0 +1,25 @@
+#!/usr/bin/env perl
+# Usage:
+#   print_c mbedtls_test_foo <file.pem
+#   print_c TEST_FOO mbedtls_test_foo <file.pem
+use strict;
+use warnings;
+
+my $pp_name = @ARGV > 1 ? shift @ARGV : undef;
+my $name = shift @ARGV;
+
+my @lines = map {chomp; s/([\\"])/\\$1/g; "\"$_\\r\\n\""} <STDIN>;
+
+if (defined $pp_name) {
+    foreach ("#define $pp_name", @lines[0..@lines-2]) {
+        printf "%-72s\\\n", $_;
+    }
+    print "$lines[@lines-1]\n";
+    print "const char $name\[\] = $pp_name;\n";
+} else {
+    print "const char $name\[\] =";
+    foreach (@lines) {
+        print "\n$_";
+    }
+    print ";\n";
+}