Use Ubuntu-20.04 runner for running header guard validation

Using `ubuntu-latest` is triggering warning that it will soon migrate
from 18.04 to 20.04, so I am migrating preventively.

.github/workflows/mac-builds.yml

@@ -11,7 +11,11 @@ jobs:
           - g++-9
           - clang++
         build_type: [Debug, Release]
-        std: [14]
+        std: [14, 17]
+        include:
+          - build_type: Debug
+            examples: ON
+            extra_tests: ON
 
     steps:
     - uses: actions/checkout@v2
@@ -24,8 +28,10 @@ jobs:
       # Note: $GITHUB_WORKSPACE is distinct from ${{runner.workspace}}.
       #       This is important
       run: |
-        cmake -Bbuild -H$GITHUB_WORKSPACE -DCMAKE_BUILD_TYPE=${{matrix.build_type}} \
-              -DCMAKE_CXX_STANDARD=${{matrix.std}} -DCATCH_DEVELOPMENT_BUILD=ON
+        cmake -Bbuild -H$GITHUB_WORKSPACE \
+              -DCMAKE_BUILD_TYPE=${{matrix.build_type}} -DCMAKE_CXX_STANDARD=${{matrix.std}} \
+              -DCATCH_DEVELOPMENT_BUILD=ON -DCATCH_BUILD_EXAMPLES=${{matrix.examples}} \
+              -DCATCH_BUILD_EXTRA_TESTS=${{matrix.examples}}
 
     - name: Build tests + lib
       working-directory: ${{runner.workspace}}/build

.github/workflows/validate-header-guards.yml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 jobs:
   build:
     # Set the type of machine to run on
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
 
       - name: Checkout source code

.travis.yml

@@ -127,13 +127,6 @@ matrix:
           packages: ['g++-7', 'lcov']
       env: COMPILER='g++-7' CPP14=1 EXAMPLES=1 COVERAGE=1 EXTRAS=1
 
-    # OSX Clang Builds
-    - os: osx
-      osx_image: xcode11.2
-      compiler: clang
-      env: COMPILER='clang++' CPP14=1 EXAMPLES=1 COVERAGE=1 EXTRAS=1
-
-
 
     # Special builds, e.g. conan
     - language: python

tests/ExtraTests/CMakeLists.txt

@@ -11,7 +11,7 @@ message( STATUS "Extra tests included" )
 # The MinDuration reporting tests do not need separate compilation, but
 # they have non-trivial execution time, so they are categorized as
 # extra tests, so that they are run less.
-add_test(NAME MinDuration::SimpleThreshold COMMAND $<TARGET_FILE:SelfTest> --min-duration 0.22 [min_duration_test])
+add_test(NAME MinDuration::SimpleThreshold COMMAND $<TARGET_FILE:SelfTest> --min-duration 0.245 [min_duration_test])
 set_tests_properties(
     MinDuration::SimpleThreshold
   PROPERTIES