[gdb/testsuite] Handle PS1 quirk in gdb.base/multi-line-starts-subshell.exp

On SLE-11, I run into:
...
(gdb) if 1^M
 >shell HOME=/dev/null PS1="gdb-subshell$ " /bin/sh^M
 >end^M
hostname:/dir> FAIL: gdb.base/multi-line-starts-subshell.exp: \
  spawn subshell from multi-line (timeout)
...

The problem is that the PS1 setting has no effect, due to a bug on older
openSUSE/SLE version.  The mechanism there is:
- /etc/profile sets ENV=/etc/bash.bashrc
- /bin/sh is started
- /bin/sh executes ENV, in other words /etc/bash.bashrc
- during the execution of /etc/bash.bashrc, PS1 is set unconditionally

Fix this by setting PS1 after spawning the subshell.

Tested on x86_64-linux.

2020-12-15  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26952
	* gdb.base/multi-line-starts-subshell.exp: Set PS1 after spawning
	shell.

1 files changed, 3 insertions, 3 deletions

diff --git a/gdb/testsuite/gdb.base/multi-line-starts-subshell.exp b/gdb/testsuite/gdb.base/multi-line-starts-subshell.exp
index 710c391..dc6d705 100644
--- a/gdb/testsuite/gdb.base/multi-line-starts-subshell.exp
+++ b/gdb/testsuite/gdb.base/multi-line-starts-subshell.exp
@@ -27,11 +27,11 @@ set shell_prompt_re [string_to_regexp $shell_prompt]
 
 # Point HOME nowhere to avoid the shell sourcing the user's init
 # scripts.
-set shell_cmd "HOME=/dev/null PS1=\"$shell_prompt\" /bin/sh"
+set shell_cmd "HOME=/dev/null /bin/sh"
 
 set test "spawn subshell from multi-line"
-gdb_test_multiple "if 1\nshell ${shell_cmd}\nend" $test {
-    -re "$shell_prompt_re$" {
+gdb_test_multiple "if 1\nshell ${shell_cmd}\nend\nPS1=\"$shell_prompt\"" $test {
+    -re "PS1=\[^\r\n\]*\r\n$shell_prompt_re$" {
 	pass $test
 
 	# Now check that shell input works and that echo is enabled.