runtests: support multi-target cmake, drop workarounds from CI

Support multi-target cmake builds via `CURL_DIRSUFFIX` env. For example:
`export CURL_DIRSUFFIX=Debug/`.

Multi-target generators place their output to `src/<subdir>/`,
`lib/<subdir>/`, `tests/server/<subdir>`, `tests/libtest/<subdir>` and
`tests/unit/<subdir>/` by default. Before this patch, `runtests.pl`
couldn't run on such builds because it expected the binaries under the
their `<subdir>`-less directories. This patch allows to set such subdir
and make `runtests.pl` find the binaries. In CI we use multi-target
builds with tests for MSVC. It also helps Xcode-generator builds, though
in CI we don't have such job running tests.

There may be better solutions to configure this, but passing a custom
value to `runtests.pl` including its subprocesses is somewhat tricky.
The reason the configuration value expects the slash at the end is
because MSYS is automagically expanding the env to a (wrong) absolute
path if the slash is in the front.

Also:
- drop the `-DCMAKE_RUNTIME_OUTPUT_DIRECTORY_*=` workaround from CI.
- replace `resolve` references in tests with a new `%RESOLVE` variable.
  It didn't use a filename extension before. After this patch it uses
  `exe_ext('TOOL')`. I'm not sure if this is the correct choice vs.
  `exe_ext('SRV')`.
- fix `-c` option format in manual.
- fix some whitespace.

Note, in CI we still tweak `CMAKE_RUNTIME_OUTPUT_DIRECTORY_*` in jobs
which share steps between `./configure` and cmake. It's easier that way.

Ref: #15000
Cherry-picked from #16394

Closes #16452

tests/http-server.pl

@@ -36,10 +36,7 @@ use File::Basename;
 use serverhelp qw(
     server_pidfilename
     server_logfilename
-    );
-
-use pathhelp qw(
-    exe_ext
+    server_exe
     );
 
 my $verbose = 0;     # set to 1 for debugging
@@ -187,8 +184,8 @@ if($ipvnum eq 'unix') {
 $flags .= "--srcdir \"$srcdir\"";
 
 if($verbose) {
-    print STDERR "RUN: server/sws".exe_ext('SRV')." $flags\n";
+    print STDERR "RUN: ".server_exe('sws')." $flags\n";
 }
 
 $| = 1;
-exec("exec server/sws".exe_ext('SRV')." $flags");
+exec("exec ".server_exe('sws')." $flags");