lib: move all UNITTEST prototypes to C files

- make extract-unit-protos handle multi-line prototypes - but they need
  to be above the implementation

- Prototypes for static functions we use in unit tests should not be in
  header files. We generate lib/unitprotos.h for this purpose

- Removed some function wrappers written for unit tests and make them
  use UNITTEST function directly.

- Renamed time2str() in the tool to timebuf() since we have the same
  name in lib/ and in unit tests they can both be used non-static in a
  build.

This reverts commit f95fadd116.

Follow-up to #21010

Closes #21014

lib/progress.c

@@ -129,6 +129,8 @@ static void pgrs_speedinit(struct Curl_easy *data)
 /*
  * @unittest: 1606
  */
+UNITTEST CURLcode pgrs_speedcheck(struct Curl_easy *data,
+                                  const struct curltime *pnow);
 UNITTEST CURLcode pgrs_speedcheck(struct Curl_easy *data,
                                   const struct curltime *pnow)
 {