Avoid the use of gdb_start_cmd in gdb.ada/*.exp testcases.

Avoid the use of gdb_start_cmd in gdb.ada/*.exp testcases.

This command does not work when testing with GDBserver.  So this patch
changes the few tests that do not specifically test the `start' command,
and replace calls to gdb_start_cmd with the usual `run LOC' approach.

For the couple of testcases that do really test the `start' command,
do an early return as UNTESTED instead of running this testcase.

gdb/testsuite/ChangeLog:

	Add marker to be used as anchor for inserting breakpoints.
	* gdb.ada/null_record/null_record.adb: Add "-- START" comment.
	* gdb.ada/start/dummy.adb, gdb.ada/uninitialized_vars/parse.adb:
	Likewise.

	Remove uses of gdb_start_cmd.
	* gdb.ada/null_record.exp: Remove use of gdb_start_cmd.
	* gdb.ada/print_pc.exp, gdb.ada/uninitialized_vars.exp: Ditto.

	Do not run testcase if testing with GDBserver.
	* gdb.ada/exec_changed.exp, gdb.ada/start.exp: Abort as untested
	if testing with GDBserver.