libstore-tests: Improve compile times with PCH

(Before)

**** Time summary:
Compilation (25 times):
  Parsing (frontend):          145.5 s
  Codegen & opts (backend):     65.9 s

(After)

**** Time summary:
Compilation (26 times):
  Parsing (frontend):           39.9 s
  Codegen & opts (backend):     55.1 s

src/libstore-tests/meson.build

@@ -94,6 +94,7 @@ this_exe = executable(
   link_args : linker_export_flags + [ '-lrapidcheck' ],
   # get main from gtest
   install : true,
+  cpp_pch : do_pch ? [ 'pch/precompiled-headers.hh' ] : [],
 )
 
 test(
@@ -127,6 +128,7 @@ if get_option('benchmarks')
     include_directories : include_dirs,
     link_args : linker_export_flags,
     install : true,
+    cpp_pch : do_pch ? [ 'pch/precompiled-headers.hh' ] : [],
     cpp_args : [
       '-DNIX_UNIT_TEST_DATA="' + meson.current_source_dir() + '/data"',
     ],

src/libstore-tests/pch/precompiled-headers.hh

@@ -0,0 +1,9 @@
+#include "nix/store/store-api.hh"
+#include "nix/store/tests/libstore.hh"
+#include "nix/util/util.hh"
+
+#include <gtest/gtest.h>
+#include <nlohmann/json.hpp>
+#include <rapidcheck/gtest.h>
+
+#include <regex>