gdb/testsuite: Merge cvexpr.exp and ctf-cvexpr.exp

gdb/testsuite: Merge cvexpr.exp and ctf-cvexpr.exp

The recently added gdb.base/ctf-cvexpr.exp is just a copy of
gdb.base/cvexpr.exp but compiled with different options.  This patch
merges these two tests together into a single test script.

I tested this change using a version of GCC with CTF support added.

gdb/testsuite/ChangeLog:

	* gdb.base/ctf-cvexpr.exp: Delete.
	* gdb.base/cvexpr.exp: Rewrite to compile as both dwarf and ctf.

Change-Id: If678c3e38cb444867defa970203d26563f15dba4