When COMPILER_PLUGIN_TOOL is set, solenv/gbuild/LinkTarget.mk uses the (plaform-
generic) gb_*__tool_command which don't know about the COMPILER_TEST flag (as
gb_CObject__command_pattern in solenv/gbuild/platform/com_GCC_class.mk does), so
a top-level Clang

  make COMPILER_PLUGIN_TOOL=...

will generate spurious warnings from CompilerTest_compilerplugins_clang's
compilations done without the -Xclang -verify switch.  This solution is a bit
hacky (e.g., it would be better to handle this more centrally in
solenv/gbuild/CompilerTest.mk), but effective.

Change-Id: I58e9a76207065d9f6050a1ace6fc85c5a26882f8