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/serverhelp.pm

@@ -45,6 +45,7 @@ BEGIN {
         server_cmdfilename
         server_inputfilename
         server_outputfilename
+        server_exe
         mainsockf_pidfilename
         mainsockf_logfilename
         datasockf_pidfilename
@@ -59,6 +60,10 @@ BEGIN {
     }
 }
 
+use globalconfig;
+use pathhelp qw(
+    exe_ext
+    );
 
 our $logfile;  # server log file name, for logmsg
 
@@ -230,6 +235,18 @@ sub server_outputfilename {
 }
 
 
+#***************************************************************************
+# Return filename for a server executable
+#
+sub server_exe {
+    my ($name, $ext) = @_;
+    if(!defined $ext) {
+        $ext = 'SRV';
+    }
+    return $SRVDIR . $name . exe_ext($ext);
+}
+
+
 #***************************************************************************
 # Return file name for main or primary sockfilter pid file.
 #