meson: add tests option

Allow the user to disable tests, for example to avoid the following
build failure on mips32:

FAILED: tests/try-syscall.p/try-syscall.c.o
/home/autobuild/autobuild/instance-11/output-1/host/bin/mipsel-buildroot-linux-musl-gcc -Itests/try-syscall.p -Itests -I../tests -fdiagnostics-color=always -Wall -Winvalid-pch -Wextra -O3 -D_GNU_SOURCE -Werror=shadow -Werror=empty-body -Werror=strict-prototypes -Werror=missing-prototypes -Werror=implicit-function-declaration -Werror=pointer-arith -Werror=init-self -Werror=missing-declarations -Werror=return-type -Werror=overflow -Werror=int-conversion -Werror=incompatible-pointer-types -Werror=misleading-indentation -Werror=missing-include-dirs -Werror=aggregate-return -Werror=switch-default -Wswitch-enum -Wno-sign-compare -Wno-error=sign-compare -Wno-missing-field-initializers -Wno-error=missing-field-initializers -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -O1 -g0 -MD -MQ tests/try-syscall.p/try-syscall.c.o -MF tests/try-syscall.p/try-syscall.c.o.d -o tests/try-syscall.p/try-syscall.c.o -c ../tests/try-syscall.c
../tests/try-syscall.c:34:5: error: #error "Unknown MIPS ABI"
   34 | #   error "Unknown MIPS ABI"
      |     ^~~~~

Fixes:
 - http://autobuild.buildroot.org/results/cf0365354fc8c16e5871d561daae0fa5039d0bee

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

meson.build

@@ -152,4 +152,6 @@ if not meson.is_subproject()
   subdir('completions')
 endif
 
-subdir('tests')
+if get_option('tests')
+  subdir('tests')
+endif

meson_options.txt

@@ -38,6 +38,12 @@ option(
   description : 'enable optional SELINUX support',
   value : 'auto',
 )
+option(
+  'tests',
+  type : 'boolean',
+  description : 'build tests',
+  value : 'true',
+)
 option(
   'zsh_completion',
   type : 'feature',