tests: use %TESTNUMBER instead of fixed number

This makes the tests easier to copy and relocate to other test numbers
without having to update content.

Closes #6738

test1918: verify curl_easy_option_by_name() and curl_easy_option_by_id()

... and as a practical side-effect, make sure that the
Curl_easyopts_check() function is asserted in debug builds

test1918: verify curl_easy_option_by_name() and curl_easy_option_by_id()

... and as a practical side-effect, make sure that the
Curl_easyopts_check() function is asserted in debug builds, which we
want to detect mismatches between the options list in easyoptions.c and
the options in curl.h

Found-by: Gisle Vanem
Bug: https://github.com/curl/curl/commit/08e8455dddc5e48e58a12ade3815c01ae3da3b64#commitcomment-45991815

Closes #6461

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