tests: do not let lazy prereqs inside `test_expect_*` turn off tracing

The `test_expect_*` functions use `test_eval_` and so does `test_run_lazy_prereq_`. If tracing is enabled via the `-x` option, `test_eval_` turns on tracing while evaluating the code block, and turns it off directly after it. This is unwanted for nested invocations. One somewhat surprising example of this is when running a test that calls `test_i18ngrep`: that function requires the `C_LOCALE_OUTPUT` prereq, and that prereq is a lazy one, so it is evaluated via `test_eval_`, the command tracing is turned off, and the test case continues to run _without tracing the commands_. Another somewhat surprising example is when one lazy prereq depends on another lazy prereq: the former will call `test_have_prereq` with the latter one, which in turn calls `test_eval_` and -- you guessed it -- tracing (if enabled) will be turned off _before_ returning to evaluating the other lazy prereq. As we will introduce just such a scenario with the GPG, GPGSM and RFC1991 prereqs, let's fix that by introducing a variable that keeps track of the current trace level: nested `test_eval_` calls will increment and then decrement the level, and only when it reaches 0, the tracing will _actually_ be turned off. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>