pw_unit_test: Make simple runner look like GTest

This makes these changes to the unit test framework:

(1) Makes the simple test runner output match GTest
(2) Adds a new RunAllTestsStart() and RunAllTestsEnd() event
(3) Adds an intentionally-failing expectation to the sample test

Example output:

[==========] Running all tests.
[ RUN      ] PigweedTest.ExpectBool
[       OK ] PigweedTest.ExpectBool
[ RUN      ] PigweedTest.ExpectBasicComparisons
[       OK ] PigweedTest.ExpectBasicComparisons
[ RUN      ] PigweedTest.ExpectStringEquality
[       OK ] PigweedTest.ExpectStringEquality
[ RUN      ] PigweedTest.NonCopyableType
[       OK ] PigweedTest.NonCopyableType
[ RUN      ] PigweedTest.MacroArgumentsOnlyAreEvaluatedOnce
[       OK ] PigweedTest.MacroArgumentsOnlyAreEvaluatedOnce
[ RUN      ] FixtureTest.CustomFixture
[       OK ] FixtureTest.CustomFixture
[ RUN      ] PigweedTestFixture.TheNumberIs35
[       OK ] PigweedTestFixture.TheNumberIs35
[ RUN      ] PigweedTestFixture.YupTheNumberIs35
[       OK ] PigweedTestFixture.YupTheNumberIs35
[ RUN      ] PigweedTestFixture.MissingExpectations
../pw_unit_test/framework_test.cc:156: Failure
      Expected: missed_expectations == 200
[  FAILED  ] PigweedTestFixture.MissingExpectations
[==========] Done running all tests.
[  PASSED  ] 8 test(s).
[  FAILED  ] 1 test(s).

Change-Id: I813096a9d62423e256993ae03724337d52702297
diff --git a/pw_build/BUILD.gn b/pw_build/BUILD.gn
index ebe935d..75003fa 100644
--- a/pw_build/BUILD.gn
+++ b/pw_build/BUILD.gn
@@ -27,6 +27,9 @@
   cflags = [
     "-Wall",
     "-Wextra",
+
+    # Warn when a switch on an enum does not cover all the cases.
+    "-Wswitch",
   ]
 }