Improve CI output. (llvm#70236)

There is currently a major problem with the CI output:

The information you need to see never appears in the visible log.

This is because our logs are very verbose, and list (A) every test as
they run it, and (B) every unsupported tests. This can be thousands of
lines.

(A) was introduced by me when I disabled the PTY progress bar, which
doesn't play nice with log files. That change was an improvement, but I
have now disabled the PTY on the builders, so we can go back to passing
`-s`. When `-s` is passed but no PTY is available, it prints a long
friendly progress indicator.

(B) is solved here by disabling the printing of unsupported tests at the
end of the test suite. While it can be useful on occasion to audit the
list of unsupported tests, it's far from a common operation. Instead
people want to see the log of their failure. We should upload the xml
results, so if auditing is needed, it can be done using that.

Hopefully this change will make it so that the test failures appear in
the actual log output

libcxx/utils/ci/run-buildbot

@@ -137,7 +137,7 @@ function generate-cmake-base() {
           -DLIBUNWIND_ENABLE_WERROR=YES \
           -DLIBCXX_ENABLE_CLANG_TIDY=${ENABLE_CLANG_TIDY} \
           ${ENABLE_STD_MODULES} \
-          -DLLVM_LIT_ARGS="-v --show-unsupported --xunit-xml-output test-results.xml --timeout=1500 --time-tests" \
+          -DLLVM_LIT_ARGS="-sv --xunit-xml-output test-results.xml --timeout=1500 --time-tests" \
           "${@}"
 }
 
@@ -372,7 +372,7 @@ bootstrapping-build)
           -DLLVM_TARGETS_TO_BUILD="host" \
           -DRUNTIMES_BUILD_ALLOW_DARWIN=ON \
           -DLLVM_ENABLE_ASSERTIONS=ON \
-          -DLLVM_LIT_ARGS="-v --show-unsupported --xunit-xml-output test-results.xml --timeout=1500 --time-tests"
+          -DLLVM_LIT_ARGS="-sv --xunit-xml-output test-results.xml --timeout=1500 --time-tests"
 
     echo "+++ Running the libc++ and libc++abi tests"
     ${NINJA} -vC "${BUILD_DIR}" check-runtimes