meson: disable long-tests by default

Some of the automated tests in this repository take a long time to
execute.  The tests can sometimes time out during CI execution, causing
a spurious failure.  The meson `long-tests` option was added to avoid
building and executing these tests during CI.

A default value of this option was previously not defined, and the long
tests were not run during CI.

With recent changes to the meson version and meson.build file, these
tests now run during CI.

Set the default value of `long-tests` to disabled so they do not run
during CI.

Change-Id: I0f749c6b23ba97dffa15a2f809f880cee22872c1
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
diff --git a/meson.options b/meson.options
index 4459290..d458978 100644
--- a/meson.options
+++ b/meson.options
@@ -17,7 +17,7 @@
     'tests', type: 'feature', description: 'Build tests.',
 )
 option(
-    'long-tests', type: 'feature',
+    'long-tests', type: 'feature', value: 'disabled',
     description: 'Build long-running tests that are excluded from CI.',
 )