[gdb/testsuite] Fix gdb.base/empty-host-env-vars.exp

On aarch64-linux (debian testing) with test-case
gdb.base/empty-host-env-vars.exp I ran into:
...
(gdb) show index-cache directory^M
The directory of the index cache is "/home/linux/.cache/gdb".^M
(gdb) FAIL: $exp: env_var_name=HOME: show index-cache directory
...

Without changing any environment variables, the value of the index-cache dir
is:
...
$ gdb -q -batch -ex "show index-cache directory"
The directory of the index cache is "/home/linux/.cache/gdb".
...
and the expectation of the test-case is that setting HOME to empty will
produce an empty dir, but what it actually produces is:
...
$ HOME= gdb -q -batch -ex "show index-cache directory"
The directory of the index cache is "/home/linux/.cache/gdb".
...

There's nothing wrong with that behaviour, the dir is simply constructed using
XDG_CACHE_HOME which happens to be explictly set to its default value
$HOME/.cache [1]:
...
$ echo $XDG_CACHE_HOME
/home/linux/.cache
...
and indeed also setting that variable to empty gets us the expected empty dir:
...
$ XDG_CACHE_HOME= HOME= gdb -q -batch -ex "show index-cache directory"
gdb: warning: Couldn't determine a path for the index cache directory.
The directory of the index cache is "".
...

Furthermore, the test-case assumption that setting variables to empty either
produces the original dir or an empty dir is incorrect.

Say that XDG_CACHE_HOME has a non-default value:
...
$ echo $XDG_CACHE_HOME
/home/linux/my-xdg-cache-home
$ gdb -q -batch -ex "show index-cache directory"
The directory of the index cache is "/home/linux/my-xdg-cache-home/gdb".
...
then setting that variable to empty:
...
$ XDG_CACHE_HOME= gdb -q -batch -ex "show index-cache directory"
The directory of the index cache is "/home/linux/.cache/gdb".
...
does change the value of the dir.

Fix this by making the test-case less specific.

While we're at it, factor out regexps re_pre and re_post to make regexps more
readable, and use string_to_regexp to reduce quoting.

Tested on aarch64-linux.

PR testsuite/32132
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32132

[1] https://specifications.freedesktop.org/basedir-spec/latest/index.html#variables

gdb/testsuite/gdb.base/empty-host-env-vars.exp

@@ -21,16 +21,14 @@ require {!is_remote host}
 
 set all_env_vars { HOME XDG_CACHE_HOME LOCALAPPDATA XDG_CONFIG_HOME }
 
-# Record the initial value of the index-cache directory.
+set re_pre \
+    [string_to_regexp {The directory of the index cache is "}]
+set re_post \
+    [string_to_regexp {".}]
+
+# Show the initial value of the index-cache directory.
 clean_restart
-set index_cache_directory ""
-gdb_test_multiple "show index-cache directory" "" {
-    -re -wrap "The directory of the index cache is \"(.*)\"\\." {
-	set index_cache_directory $expect_out(1,string)
-	set index_cache_directory [string_to_regexp $index_cache_directory]
-	pass $gdb_test_name
-    }
-}
+gdb_test "show index-cache directory" $re_pre\[^\r\n\]*$re_post
 
 foreach_with_prefix env_var_name $all_env_vars {
     # Restore the original state of the environment variable.
@@ -38,18 +36,7 @@ foreach_with_prefix env_var_name $all_env_vars {
 	set env($env_var_name) {}
 	clean_restart
 
-	# Verify that the empty environment variable didn't affect the
-	# index-cache directory setting, that we still see the initial value.
-	# "HOME" is different, because if that  one is unset, GDB isn't even
-	# able to compute the default location.  In that case, we expect it to
-	# be empty.
-	if { $env_var_name == "HOME" } {
-	    gdb_test "show index-cache directory" \
-		"The directory of the index cache is \"\"\\."
-	} else {
-	    gdb_test "show index-cache directory" \
-		"The directory of the index cache is \"$index_cache_directory\"\\."
-	}
+	gdb_test "show index-cache directory" $re_pre\[^\r\n\]*$re_post
     }
 }
 
@@ -69,7 +56,6 @@ with_test_prefix "all env vars" {
 
 	clean_restart
 
-	gdb_test "show index-cache directory" \
-	    "The directory of the index cache is \"\"\\."
+	gdb_test "show index-cache directory" $re_pre$re_post
     }
 }