tests: make first.c the same for both lib tests and unit tests

The only difference used to be global variable used in unittest tests.
After cb7ed5a removed individual flag overrides for the unittests, first.c
was no longer recompiled for unit tests to include the flag, so whether it
worked or gave a link error depended on whether it was compiled in
libtest or unittest first. This way also speeds up the build by
eliminating 40 identical compile invocations.

Fixes #10749

tests/libtest/first.c

@@ -85,9 +85,7 @@ char **test_argv;
 
 struct timeval tv_test_start; /* for test timing */
 
-#ifdef UNITTESTS
 int unitfail; /* for unittests */
-#endif
 
 #ifdef CURLDEBUG
 static void memory_tracking_init(void)

tests/libtest/test.h

@@ -73,9 +73,7 @@ extern int test(char *URL); /* the actual test function provided by each
 
 extern char *hexdump(const unsigned char *buffer, size_t len);
 
-#ifdef UNITTESTS
 extern int unitfail;
-#endif
 
 /*
 ** TEST_ERR_* values must be greater than CURL_LAST CURLcode in order