[Tests] Update comments (NFC)

Spruce up the code comments in the tests' lit.cfg. Mirror the
work in swiftlang/swift-corelibs-foundation#431,
which removes references to "OS X", plus make a few parts slightly more
readable.

Author: modocache

Tests/Functional/lit.cfg

@@ -46,7 +46,7 @@ swift_exec = [
 ]
 
 if platform.system() == 'Darwin':
-    # On OS X, we need to make sure swiftc references the
+    # On Darwin, we need to make sure swiftc references the
     # proper SDK, has a deployment target set, and more...
     # Here we rely on environment variables, produced by xcodebuild.
     sdk_root = _getenv('SDKROOT')
@@ -77,7 +77,8 @@ else:
         '-I', core_foundation_dir,
     ])
     config.environment['LD_LIBRARY_PATH'] = foundation_dir
-    # as well as libdispatch if Foundation is using it
+    # We also need to link swift-corelibs-libdispatch, if
+    # swift-corelibs-foundation is using it.
     libdispatch_src_dir = os.getenv('LIBDISPATCH_SRC_DIR')
     libdispatch_build_dir = os.getenv('LIBDISPATCH_BUILD_DIR')
     if (libdispatch_src_dir is not None) and (libdispatch_build_dir is not None):
@@ -90,7 +91,7 @@ else:
 # Having prepared the swiftc command, we set the substitution.
 config.substitutions.append(('%{swiftc}', ' '.join(swift_exec)))
 
-# Add the %xctest_checker substitution, which is a Python script
+# Add the %{xctest_checker} substitution, which is a Python script that
 # can be used to compare the actual XCTest output to the expected
 # output.
 xctest_checker = os.path.join(