Update tests.md with example

[Su24nn3]

Added example about where to find the line to move past the next test.

[vaeng]

Hello @apprentiLAB-Suzanne,

thanks for your PR. It is a good thing to show an actual file. The bob exercise might be unknown, so I would show it and not make the person search for the source code of the bob test. Probably something like:

To include the next test you have to move the line with if defined(EXERCISM_RUN_ALL_TESTS) past the desired test.
See the example below from the Bob exercise:

-#if defined(EXERCISM_RUN_ALL_TESTS)
TEST_CASE("shouting")
{
    REQUIRE("Whoa, chill out!" == bob::hey("WATCH OUT!"));
}
+#if defined(EXERCISM_RUN_ALL_TESTS)

[Su24nn3]

Hi @vaeng,
thanks for the suggestion, I updated the PR !
I kept "next test", instead of "desired test". I find that it might be simpler for beginners, if you prefer otherwise I can change it 🙂
Cheers

[vaeng]

LGTM!