[gdb/testsuite] Add -wrap pattern flag to gdb_test_multiple

[gdb/testsuite] Add -wrap pattern flag to gdb_test_multiple

Currently, in order to rewrite:
...
gdb_test <command> <pattern> <message>
...
using gdb_test_multiple, we get:
...
gdb_test_multiple <command> <message> {
    -re "\[\r\n\]*(?:<pattern>)\[\r\n\]+$gdb_prompt $" {
    	pass $gdb_test_name
    }
}
...

Add a '-wrap pattern flag to gdb_test_multiple, that wraps the regexp
pattern as gdb_test wraps its message argument.

This allows us to rewrite into the more compact:
...
gdb_test_multiple <command> <message> {
    -re -wrap <pattern> {
        pass $gdb_test_name
    }
}
...

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-10-24  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (gdb_test_multiple): Add -wrap pattern flag.
	* gdb.reverse/step-precsave.exp: Rewrite gdb_test_multiple containing
	kfail using -wrap pattern flag and convenience variable
	gdb_test_name.

Change-Id: Ie42c97d5ab7acf6db351299ccd23a83540fe6e1a