cmake: fix duplicate symbols when linking tests

[vszakats]

The linker resolves this automatically in non-unity builds. In unity builds the linker cannot drop a single object with the duplicates, resulting in these errors. The root issue is that we started including certain objects both via both libcurlu and libcurltool libs.

Regression from 39e7c22

Windows errors:

[  3%] Linking C executable unit1303.exe
[  3%] Building C object tests/server/CMakeFiles/rtspd.dir/__/__/lib/curl_multibyte.c.obj
../../lib/libcurlu-d.a(unity_0.c.obj): In function `curlx_convert_UTF8_to_wchar':
C:/projects/curl/lib/curl_multibyte.c:44: multiple definition of `curlx_convert_UTF8_to_wchar'
../../src/libcurltool-d.a(unity_0.c.obj):C:/projects/curl/lib/curl_multibyte.c:44: first defined here
../../lib/libcurlu-d.a(unity_0.c.obj): In function `curlx_convert_wchar_to_UTF8':
C:/projects/curl/lib/curl_multibyte.c:66: multiple definition of `curlx_convert_wchar_to_UTF8'
../../src/libcurltool-d.a(unity_0.c.obj):C:/projects/curl/lib/curl_multibyte.c:66: first defined here
../../lib/libcurlu-d.a(unity_0.c.obj): In function `curlx_win32_open':
C:/projects/curl/lib/curl_multibyte.c:92: multiple definition of `curlx_win32_open'
../../src/libcurltool-d.a(unity_0.c.obj):C:/projects/curl/lib/curl_multibyte.c:92: first defined here
../../lib/libcurlu-d.a(unity_0.c.obj): In function `curlx_win32_fopen':
C:/projects/curl/lib/curl_multibyte.c:120: multiple definition of `curlx_win32_fopen'
../../src/libcurltool-d.a(unity_0.c.obj):C:/projects/curl/lib/curl_multibyte.c:120: first defined here
../../lib/libcurlu-d.a(unity_0.c.obj): In function `curlx_win32_stat':
[...]

Ref: https://ci.appveyor.com/project/curlorg/curl/builds/48110107/job/nvlhpt9aa4ehny5q#L247

macOS errors:

[ 56%] Linking C executable unit1302
duplicate symbol '_curlx_sotouz' in:
    ../../lib/libcurlu.a(unity_0_c.c.o)
    ../../src/libcurltool.a(unity_0_c.c.o)
duplicate symbol '_curlx_sitouz' in:
    ../../lib/libcurlu.a(unity_0_c.c.o)
    ../../src/libcurltool.a(unity_0_c.c.o)
duplicate symbol '_curlx_uztosz' in:
    ../../lib/libcurlu.a(unity_0_c.c.o)
    ../../src/libcurltool.a(unity_0_c.c.o)
[...]

with config:

  -DCMAKE_UNITY_BUILD=ON \
  -DENABLE_DEBUG=ON -DBUILD_TESTING=ON -DCMAKE_C_FLAGS=-DDEBUGBUILD \
  -DBUILD_SHARED_LIBS=ON \
  -DBUILD_STATIC_LIBS=OFF

Closes #11926