[gdb/testsuite] Fix check-read1 FAIL in gdb.tui/corefile-run.exp

With test-case gdb.tui/corefile-run.exp and make target check-read1, I run
into:
...
FAIL: gdb.tui/corefile-run.exp: run until the end
...

In more detail, using -v:
...
PASS: gdb.tui/corefile-run.exp: load corefile
^M+++ _ctl_0x0d
^[[17d+++ _csi_d <<<17>>>
^[[M+++ _csi_M <<<>>>
^[[24d+++ _csi_d <<<24>>>
(INSERT <<(>>
gINSERT <<g>>
dINSERT <<d>>
bINSERT <<b>>
)INSERT <<)>>
 INSERT << >>
FAIL: gdb.tui/corefile-run.exp: run until the end
...

With some debugging code added in wait_for, what happens becomes more clear:
...
	    if {[regexp -- $wait_for $prev]} {
+		verbose -log "\nwait_for: MATCHED line ($_cur_y): \"$prev\""
+		verbose -log "wait_for: AGAINST regexp: \"$wait_for\""
...

In corefile-run.exp, we execute:
...
Term::command "run"
...
and in proc Term::command, we send the command, and then call wait_for:
...
    proc command {cmd} {
        send_gdb "$cmd\n"
        wait_for [string_to_regexp $cmd]
    }
...
which first waits for the command string, and then for the prompt.

In this case however, the matching of the command string triggers on a
previous line:
...
wait_for: MATCHED line (16): \
  "(gdb) core-file corefile-run.core[New LWP 6426] <lots-of-spaces>"
wait_for: AGAINST regexp: "run"
...
and from there on things go out of sync, eventually resulting in the FAIL.

Fix this in proc command by more precisely specifying the expected pattern:
adding a ^$gdb_prompt prefix.

Add a command_no_prompt_prefix variant to use for initial terminal commands
where there's no prompt yet.

Tested gdb.tui/*.exp on x86_64-linux, with make target check and check-read1.

gdb/testsuite/ChangeLog:

2020-03-13  Tom de Vries  <tdevries@suse.de>

	* lib/tuiterm.exp (Term::command_no_prompt_prefix): New proc.
	(Term::command): Use prompt prefix.
	(Term::enter_tui): Use command_no_prompt_prefix instead of prefix.
	* gdb.tui/tui-layout-asm-short-prog.exp: Use
	command_no_prompt_prefix instead of prefix.
	* gdb.tui/tui-layout-asm.exp: Same.