tests: Added numerous tests.

The tests are taken almost verbatim from the open source project PicoC. It can
be found at https://code.google.com/p/picoc/.

The tests range from very simple/trivial ones to more complicated. My view is
that the more tests the better. Without tests like this I was very reluctant to
make any changes to tcc for the fear of breaking things.

The tests pass on Win32, OSX, Linux x86 and x86_64. One or two tests fail on
each platform due to differences in the runtime library.

tests2/03_struct.c

@@ -0,0 +1,31 @@
+#include <stdio.h>
+
+struct fred
+{
+   int boris;
+   int natasha;
+};
+
+void main() 
+{
+   struct fred bloggs;
+
+   bloggs.boris = 12;
+   bloggs.natasha = 34;
+
+   printf("%d\n", bloggs.boris);
+   printf("%d\n", bloggs.natasha);
+
+   struct fred jones[2];
+   jones[0].boris = 12;
+   jones[0].natasha = 34;
+   jones[1].boris = 56;
+   jones[1].natasha = 78;
+
+   printf("%d\n", jones[0].boris);
+   printf("%d\n", jones[0].natasha);
+   printf("%d\n", jones[1].boris);
+   printf("%d\n", jones[1].natasha);
+
+   return 0;
+}