I am developing an app in such an android device that requires each app to be signed with a specific key, even the testing apk.
I know how to sign an app by configuring build.gradle. But signing testing app (Instrumentation test) seems no such configing, am I missing something ?
Thanks for NateZh and TWL.
Yes, the answer is to add flowing lines to build.gradle:
signingConfigs {
    debug {
        keyPassword 'your pw'
        storeFile file('your keystore file path')
        storePassword 'your pw'
        keyAlias 'your alias'
    }
}
But, there are more to be take care of:
When using signingConfigs.debug, this tells gradle it's debug key, no need to specific  buildTypes.debug.signingConfig again
SigingConfigs.debug also take effect when building instrumentation test
In multi-modules project, if you want to test a module, you should include signingConfigs.debug in the module's build.gradle. 
For example, my project looks like:
app
    -build.gradle
libs
    -src/androidTest/java/com/my/MyTest.java
    -build.gradle
Adding signingConfigs.debug to app has no effect when I want to run libs' AndroidDebugTest. Instead, adding signingConfigs.debug to libs' build.gradle do the work.
Hope it's helpful to others.
try this in build.gradle
signingConfigs {
    release {
        keyPassword 'your pw'
        storeFile file('your keystore file path')
        storePassword 'your pw'
        keyAlias 'your alias'
    }
}
buildTypes {
    release {
        signingConfig signingConfigs.release
    }
    debug {
        minifyEnabled false
        signingConfig signingConfigs.release
    }
}
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