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.

tests2/19_pointer_arithmetic.c

@@ -0,0 +1,28 @@
+#include <stdio.h>
+
+int main()
+{
+   int a;
+   int *b;
+   int *c;
+
+   a = 42;
+   b = &a;
+   c = NULL;
+
+   printf("%d\n", *b);
+
+   if (b == NULL)
+      printf("b is NULL\n");
+   else
+      printf("b is not NULL\n");
+
+   if (c == NULL)
+      printf("c is NULL\n");
+   else
+      printf("c is not NULL\n");
+
+   return 0;
+}
+
+/* vim: set expandtab ts=4 sw=3 sts=3 tw=80 :*/