test: make atomicity setup conditional on gdb being present.

The setup is useless if gdb is not present, so it doesn't hurt to skip
it.  The diff here is huge, but the commit is really just moving most
of the script inside the initial if, and adding an else block to print
a warning.

test/atomicity

@@ -7,9 +7,8 @@ test_description='atomicity'
 # final database contents should be the same regardless of when (or
 # if) it is killed and restarted.
 
-if test_expect_success "prereq: GDB is present" "which gdb"; then
+if which gdb; then
     test_set_prereq GDB
-fi
 
 # Create a maildir structure to also stress flag synchronization
     mkdir $MAIL_DIR/cur
@@ -92,6 +91,10 @@ for ((i = 0; i < $outcount; i++)); do
 	    i=$(expr $end - 1)
 	fi
     done
+else
+    say_color info "%-6s" "WARNING"
+    echo " Missing test prerequisite GDB"
+fi  
 
 test_begin_subtest '"notmuch new" is idempotent under arbitrary aborts'
 test_expect_equal_file GDB searchall expectall