[gdb/testsuite] Fix gdb.mi/mi-dprintf.exp with read1

[gdb/testsuite] Fix gdb.mi/mi-dprintf.exp with read1

When running test-case gdb.mi/mi-dprintf.exp with check-read1, I run into:
...
(gdb) ^M
PASS: gdb.mi/mi-dprintf.exp: gdb: mi 2nd dprintf stop
-data-evaluate-expression stderr^M
^done,value="0x7ffff7e4a420 <_IO_2_1_stderr_>"^M
(gdb) FAIL: gdb.mi/mi-dprintf.exp: stderr symbol check
...

The problem is in proc mi_gdb_is_stderr_available:
...
proc mi_gdb_is_stderr_available {} {
    set has_stderr_symbol false
    gdb_test_multiple "-data-evaluate-expression stderr" "stderr symbol check" {
	-re "\\^error,msg=\"'stderr' has unknown type; cast it to its declared type\"\r\n$::mi_gdb_prompt$" {
	}
	-re "$::mi_gdb_prompt$" {
	    set has_stderr_symbol true
	}
     }
...
which uses a gdb_test_multiple that is supposed to use the mi prompt, but
doesn't use -prompt to indicate this.  Consequently, the default patterns use
the regular gdb prompt, which trigger earlier than the two custom patterns
which use "$::mi_gdb_prompt$".

Fix this by adding the missing -prompt "$::mi_gdb_prompt$" arguments.

While we're at it, make the gdb_test_multiple call a bit more readable by
using variables, and by using -wrap.

Tested on x86_64-linux, with:
- gcc and clang (to trigger both the has_stderr_symbol true and false cases)
- make check and make check-read1.