[gdb/testsuite] Fix mi-sym-info.exp with check-read1

When running gdb.mi/mi-sym-info.exp with check-read1, we run into:
...
FAIL: gdb.mi/mi-sym-info.exp: List all functions
FAIL: gdb.mi/mi-sym-info.exp: List all variables
...

The problem is that while the $mi_gdb_prompt is active, gdb_test_multiple is
used without -prompt "$mi_gdb_prompt$", so it defaults to matching $gdb_prompt.

Fix this by adding the missing gdb_test_multiple arguments.

Reg-tested on x86_64-linux with make targets check and check-read1.

gdb/testsuite/ChangeLog:

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

	* gdb.mi/mi-sym-info.exp: Add missing -prompt "$mi_gdb_prompt$" to
	gdb_test_multiple calls.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-03-03  Tom de Vries  <tdevries@suse.de>
+
+	* gdb.mi/mi-sym-info.exp: Add missing -prompt "$mi_gdb_prompt$" to
+	gdb_test_multiple calls.
+
 2020-03-02  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* gdb.base/default.exp: Remove test of 'set history filename'.

gdb/testsuite/gdb.mi/mi-sym-info.exp

@@ -68,7 +68,8 @@ mi_gdb_test "113-symbol-info-types" \
 # this command that we overflow expect's buffers, avoid this by
 # fetching the output piece by piece.
 set testname "List all functions"
-gdb_test_multiple "114-symbol-info-functions --include-nondebug" ${testname} {
+set cmd "114-symbol-info-functions --include-nondebug"
+gdb_test_multiple $cmd ${testname} -prompt "${mi_gdb_prompt}$" {
     -re "114\\^done,symbols=\{debug=\\\[${symtab_re}(?:,${symtab_re})*\\\],nondebug=\\\[" {
 	exp_continue
     }
@@ -83,7 +84,8 @@ gdb_test_multiple "114-symbol-info-functions --include-nondebug" ${testname} {
 }
 
 set testname "List all variables"
-gdb_test_multiple "115-symbol-info-variables --include-nondebug" ${testname} {
+set cmd "115-symbol-info-variables --include-nondebug"
+gdb_test_multiple $cmd ${testname} -prompt "${mi_gdb_prompt}$" {
     -re "115\\^done,symbols=\{debug=\\\[${symtab_re}(?:,${symtab_re})*\\\],nondebug=\\\[" {
 	verbose -log "Got the first part of the input"
 	exp_continue