tfile.exp: Test printing a non-const global that is not covered by the trace frame...

tfile.exp: Test printing a non-const global that is not covered by the trace frame; test disassembling.

Make sure we don't fallback to printing the initial value of a
non-const variable in the executable.

Also make sure we can do 'disassemble', as another test that GDB is
able to read read-only parts from the executable (the existing test of
printing constglob also covers that case).

gdb/testsuite/
2013-03-20  Pedro Alves  <palves@redhat.com>
	    Yao Qi  <yao@codesourcery.com>

	* gdb.trace/tfile.c: Add comments.
	(nonconstglob): New global.
	* gdb.trace/tfile.exp: Add comments.  Test printing a non-const
	global that is not covered by the trace frame.  Test
	disassembling.