[gdb/testsuite] Fix another fail and tcl error in gdb.dap/sources.exp

With gdb.dap/sources.exp on aarch64-linux, I'm running into:
...
{"request_seq": 3, "type": "response", "command": "loadedSources", \
  "success": false, "message": "notStopped", "seq": 7}Content-Length: 92^M
^M
{"type": "event", "event": "thread", \
  "body": {"reason": "started", "threadId": 1}, \
  "seq": 8}FAIL: gdb.dap/sources.exp: loadedSources success
ERROR: tcl error sourcing gdb.dap/sources.exp.
ERROR: tcl error code TCL LOOKUP DICT body
ERROR: key "body" not known in dictionary
    while executing
"dict get [lindex $obj 0] body sources"
...

These are the same type of tcl error and FAIL I just fixed for a later
request in the same test-case.

Fix this by:
- moving the wait-for-stop to before the loadedSources request to fix the
  FAIL, and
- checking for $obj == "" to fix the tcl error.

Also make the code a bit less indented and more readable by wrapping the tests
in a proc, allowing the use of return to bail out, while still running
dap_shutdown afterwards.

Approved-By: Tom Tromey <tom@tromey.com>

Tested on aarch64-linux.

gdb/testsuite/gdb.dap/sources.exp

@@ -33,30 +33,37 @@ if {[dap_launch $testfile stop_at_main 1] == ""} {
     return
 }
 
-set obj [dap_check_request_and_response loadedSources loadedSources]
+proc do_tests {} {
-set path ""
-foreach src [dict get [lindex $obj 0] body sources] {
-    if {[file tail [dict get $src name]] == "sources.c"} {
-	set path [dict get $src path]
-    }	
-}
-
-if {$path == ""} {
-    fail "sources.c in loadedSources"
-} else {
-    pass "sources.c in loadedSources"
-
     dap_wait_for_event_and_check "stopped at function breakpoint" stopped \
 	"body reason" breakpoint
 
+    set obj [dap_check_request_and_response loadedSources loadedSources]
+    if { $obj == "" } {
+	return
+    }
+
+    set path ""
+    foreach src [dict get [lindex $obj 0] body sources] {
+	if {[file tail [dict get $src name]] == "sources.c"} {
+	    set path [dict get $src path]
+	}
+    }
+    gdb_assert {$path != "" } "sources.c in loadedSources"
+    if {$path == ""} {
+	return
+    }
+
     set obj [dap_check_request_and_response "get source" source \
 		 [format {o source [o path [s %s]] \
-			    sourceReference [i 0]} $path]]
+			      sourceReference [i 0]} $path]]
-    if { $obj != "" } {
+    if { $obj == "" } {
-	set text [dict get [lindex $obj 0] body content]
+	return
-	gdb_assert {[string first "Distinguishing comment" $text] != -1}
     }
+
+    set text [dict get [lindex $obj 0] body content]
+    gdb_assert {[string first "Distinguishing comment" $text] != -1}
 }
 
+do_tests
 
 dap_shutdown