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.

tests2/16_nesting.expect

@@ -0,0 +1,18 @@
+0 0 0
+0 0 1
+0 0 2
+0 1 0
+0 1 1
+0 1 2
+0 2 0
+0 2 1
+0 2 2
+1 0 0
+1 0 1
+1 0 2
+1 1 0
+1 1 1
+1 1 2
+1 2 0
+1 2 1
+1 2 2