tests: Automatically deduce the tool name from the test case for unit tests

It is still possible to override the executable to run during the test,
using the <tool> tag, but this patch removes the requirement that the
tag must be present for unit tests.

It also removes the possibility of human error when existing test cases
are used as the basis for new tests, as recently witnessed in 81c37124.

Reviewed-by: Daniel Stenberg
Closes #4976

tests/data/test1302

@@ -18,9 +18,5 @@ unittest
  <name>
 base64 encode/decode unit tests
  </name>
-<tool>
-unit1302
-</tool>
 </client>
-
 </testcase>