meson: Default tests feature to 'auto'

With that, reduce the strength of the get_option('tests') conditions to
allowed().

Change-Id: I2ae42606a64d39c2bc5667bf009c95558c0bc4d6
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/meson.options b/meson.options
index 57d922a..dcc1fdd 100644
--- a/meson.options
+++ b/meson.options
@@ -1,4 +1,4 @@
 option('abi', type: 'array', description: 'Constrain exposed symbol classes', choices: ['deprecated', 'stable', 'testing'], value: ['deprecated', 'stable', 'testing'])
-option('tests', type: 'feature', description: 'Build tests', value: 'enabled')
+option('tests', type: 'feature', description: 'Build tests')
 option('oem-ibm', type: 'feature', description: 'Enable IBM OEM PLDM', yield: true)
 option('abi-compliance-check', type: 'feature', description: 'Detect public ABI/API changes')