Fix gdb.base/starti.exp racy test

Fix gdb.base/starti.exp racy test

This commit fixes a couple problems with gdb.base/starti.exp, causing
spurious FAILs.

The first is a double-prompt problem:

~~~
 (gdb) PASS: gdb.base/starti.exp: hook-stop
 starti
 [....]
 gdb_expect_list pattern: /\$1 = 0/
 $1 = 0

 gdb_expect_list pattern: //
 0x00007ffff7ddcc80 in _start () from /lib64/ld-linux-x86-64.so.2

 (gdb)                                         # EXPECTED PROMPT
 (gdb) PASS: gdb.base/starti.exp: starti       # ANOTHER PROMPT!
 break main
~~~

This happens because the test uses gdb_test_sequence with no command,
like this:

 gdb_test_sequence "" "starti" {
     "Program stopped."
     "\\$1 = 0"
 }

but gdb_test_sequence doesn't have a check for empty command like
gdb_test_multiple does, and so sends "\n" to GDB:

 proc gdb_test_sequence { command test_name expected_output_list } {
     global gdb_prompt
     if { $test_name == "" } {
	 set test_name $command
     }
     lappend expected_output_list ""; # implicit ".*" before gdb prompt
     send_gdb "$command\n"
     return [gdb_expect_list $test_name "$gdb_prompt $" $expected_output_list]
 }

"starti" is a no-repeat command, so pressing <ret> just makes another
prompt appear, confusing the following gdb_test/gdb_test_multiple/etc.

Even with that fixed, the testcase is still racy though.

The second problem is that sometimes the "continue" test times out
here:

~~~
 continue
 Continuing.
 $2 = 1

 gdb_expect_list pattern: /.*Breakpoint .*main \(\) at .*starti.c.*/
 Breakpoint 1, main () at /home/pedro/src/gdb/testsuite/gdb.base/starti.c:29
 29	  return 0;
 (gdb) gdb_expect_list pattern: //
 * hung here *
~~~

The problem is that the too-greedy ".*" trailing match in
gdb_expect_list's pattern ends up consuming GDB's prompt too soon.
Fix that by removing the unnecessary trailing ".*".  While at it,
remove all ".*"s to be stricter.

Tested on x86_64 GNU/Linux.

gdb/testsuite/ChangeLog:
2017-11-16  Pedro Alves  <palves@redhat.com>

	* gdb.base/starti.exp ("continue" test): Remove ".*"s from
	pattern.
	* lib/gdb.exp (gdb_test_sequence): Don't send empty command to
	GDB.