Move tests into Carbon::Testing, set small size (#992)

The small size is for the 1m vs 5m time limit -- all these tests _should_ be fast so a lower limit seems consistent, and the 5m timeout was getting in my way when trying to debug *actual* timeouts.

The Carbon::Testing bit is for convenience -- test libraries are generally using it, it seems like the tests should too. Note this reduces the need for `using`.

This does push NodeMatchers into Carbon::Testing -- I don't think this was benefiting from having its own namespace; `using namespace` is discouraged [under style](https://google.github.io/styleguide/cppguide.html#Namespaces), we wouldn't support an equivalent in Carbon, and it feels like it's not helping to avoid name collisions. (also tidy was bugging about it, and while I could NOLINT that, this felt like the better approach)

toolchain/source/source_buffer_test.cpp

@@ -11,7 +11,7 @@
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/raw_ostream.h"
 
-namespace Carbon {
+namespace Carbon::Testing {
 namespace {
 
 TEST(SourceBufferTest, StringRep) {
@@ -59,4 +59,4 @@ TEST(SourceBufferTest, FileRep) {
 }
 
 }  // namespace
-}  // namespace Carbon
+}  // namespace Carbon::Testing