gdb: Merge similar tests into a single test script

gdb: Merge similar tests into a single test script

The three test scripts:

  gdb/testsuite/gdb.base/vla-optimized-out.exp
  gdb/testsuite/gdb.base/vla-optimized-out-o3.exp
  gdb/testsuite/gdb.base/vla-optimized-out-o3-strict.exp

are all pretty similar, with differences in the compile flags used,
and some of the expected results.

Instead of maintaining 3 files, merge them into a single test script,
and use parameters to control the test behaviour.

gdb/testsuite/ChangeLog:

	* gdb.base/vla-optimized-out-o3.exp: Delete.
	* gdb.base/vla-optimized-out-o3-strict.exp: Delete.
	* gdb.base/vla-optimized-out.exp: Extend to cover all of the
	deleted tests.